Disappearing genes: a paper is refuted before it is even published

Several readers alerted me to a paper that was posted on bioRxiv a few weeks ago (May 28, 2018). The paper claimed that the human genome contains 43,162 genes consisting of 21,306 protein-coding genes and 21,856 noncoding genes. The authors reported that they had discovered 3,819 new noncoding genes and 1,178 new protein-coding genes. In addition, they claim to have discovered 97,511 new splice variants raising the total number of splice variants to 12.5 per protein-coding gene although they seem to suggest that almost one-third of these splice variants are non-functional splicing errors. The most striking result, according to the authors, is that 95% of all transcripts are just transcriptional noise.

Here's the paper ...

Press release from the Francis Crick Institute misrepresents junk DNA

Press releases have become a serious problem. I'm frequently upset whenever I read a press release covering a field I'm familiar with. They rarely do a good job of explaining what's actually in the paper and putting it into the proper context. The people who write press releases are more concerned with sensationalizing the work than they are with teaching the general public about how science works. They often do this with the blessing and participation of the scientists who did the work.

Let me illustrate the problem using a recent examples from the Francis Crick Institute in London, UK [Non-coding DNA changes the genitals you're born with]. The press release covers a recent Science paper from the Lovell-Badge lab ....

Required reading for the junk DNA debate

This is a list of scientific papers on junk DNA that you need to read (and understand) in order to participate in the junk DNA debate. It's not a comprehensive list because it's mostly papers that defend junk DNA and refute arguments for massive amounts of function. The only exception is the paper by Mattick and Dinger (2013).¹ It's the only anti-junk paper that attempts to deal with the main evidence for junk DNA. If you know of any other papers that make a good case against junk DNA then I'd be happy to include them in the list.

If you come across a publication that argues against junk DNA, then you should immediately check the reference list. If you do not see some of these references in the list, then don't bother reading the paper because you know the author is not knowledgeable about the subject.

Brenner, S. (1998) Refuge of spandrels. Current Biology, 8:R669-R669. [PDF]

Brunet, T.D., and Doolittle, W.F. (2014) Getting “function” right. Proceedings of the National Academy of Sciences, 111:E3365-E3365. [doi: 10.1073/pnas.1409762111]

Casane, D., Fumey, J., et Laurenti, P. (2015) L’apophénie d’ENCODE ou Pangloss examine le génome humain. Med. Sci. (Paris) 31: 680-686. [doi: 10.1051/medsci/20153106023] [The apophenia of ENCODE or Pangloss looks at the human genome]

Cavalier-Smith, T. (1978) Nuclear volume control by nucleoskeletal DNA, selection for cell volume and cell growth rate, and the solution of the DNA C-value paradox. Journal of Cell Science, 34(1), 247-278. [doi: PDF]

Doolittle, W.F. (2013) Is junk DNA bunk? A critique of ENCODE. Proc. Natl. Acad. Sci. (USA) published online March 11, 2013. [PubMed] [doi: 10.1073/pnas.1221376110]

Doolittle, W.F., Brunet, T.D., Linquist, S., and Gregory, T.R. (2014) Distinguishing between “function” and “effect” in genome biology. Genome biology and evolution 6, 1234-1237. [doi: 10.1093/gbe/evu098]

Doolittle, W.F., and Brunet, T.D. (2017) On causal roles and selected effects: our genome is mostly junk. BMC biology, 15:116. [doi: 10.1186/s12915-017-0460-9]

Eddy, S.R. (2012) The C-value paradox, junk DNA and ENCODE. Current Biology, 22:R898. [doi: 10.1016/j.cub.2012.10.002]

Eddy, S.R. (2013) The ENCODE project: missteps overshadowing a success. Current Biology, 23:R259-R261. [10.1016/j.cub.2013.03.023]

Graur, D. (2017) Rubbish DNA: The functionless fraction of the human genome Evolution of the Human Genome I (pp. 19-60): Springer. [doi: 10.1007/978-4-431-56603-8_2 (book)] [PDF]

Graur, D. (2017) An upper limit on the functional fraction of the human genome. Genome Biology and Evolution, 9:1880-1885. [doi: 10.1093/gbe/evx121]

Graur, D., Zheng, Y., Price, N., Azevedo, R. B., Zufall, R. A., and Elhaik, E. (2013) On the immortality of television sets: "function" in the human genome according to the evolution-free gospel of ENCODE. Genome Biology and Evolution published online: February 20, 2013 [doi: 10.1093/gbe/evt028

Graur, D., Zheng, Y., and Azevedo, R.B. (2015) An evolutionary classification of genomic function. Genome Biology and Evolution, 7:642-645. [doi: 10.1093/gbe/evv021]

Gregory, T. R. (2005) Synergy between sequence and size in large-scale genomics. Nature Reviews Genetics, 6:699-708. [doi: 10.1038/nrg1674]

Haerty, W., and Ponting, C.P. (2014) No Gene in the Genome Makes Sense Except in the Light of Evolution. Annual review of genomics and human genetics, 15:71-92. [doi:10.1146/annurev-genom-090413-025621]

Hurst, L.D. (2013) Open questions: A logic (or lack thereof) of genome organization. BMC biology, 11:58. [doi:10.1186/1741-7007-11-58]

Kellis, M., Wold, B., Snyder, M.P., Bernstein, B.E., Kundaje, A., Marinov, G.K., Ward, L.D., Birney, E., Crawford, G. E., and Dekker, J. (2014) Defining functional DNA elements in the human genome. Proc. Natl. Acad. Sci. (USA) 111:6131-6138. [doi: 10.1073/pnas.1318948111]

Mattick, J. S., and Dinger, M. E. (2013) The extent of functionality in the human genome. The HUGO Journal, 7:2. [doi: 10.1186/1877-6566-7-2]

Five Things You Should Know if You Want to Participate in the Junk DNA DebateMorange, M. (2014) Genome as a Multipurpose Structure Built by Evolution. Perspectives in biology and medicine, 57:162-171. [doi: 10.1353/pbm.2014.000]

Niu, D. K., and Jiang, L. (2012) Can ENCODE tell us how much junk DNA we carry in our genome?. Biochemical and biophysical research communications 430:1340-1343. [doi: 10.1016/j.bbrc.2012.12.074]

Ohno, S. (1972) An argument for the genetic simplicity of man and other mammals. Journal of Human Evolution, 1:651-662. [doi: 10.1016/0047-2484(72)90011-5]

Ohno, S. (1972) So much "junk" in our genome. In H. H. Smith (Ed.), Evolution of genetic systems (Vol. 23, pp. 366-370): Brookhaven symposia in biology.

Palazzo, A.F., and Gregory, T.R. (2014) The Case for Junk DNA. PLoS Genetics, 10:e1004351. [doi: 10.1371/journal.pgen.1004351]

Rands, C. M., Meader, S., Ponting, C. P., and Lunter, G. (2014) 8.2% of the Human Genome Is Constrained: Variation in Rates of Turnover across Functional Element Classes in the Human Lineage. PLOS Genetics, 10:e1004525. [doi: 10.1371/journal.pgen.1004525]

Thomas Jr, C.A. (1971) The genetic organization of chromosomes. Annual review of genetics, 5:237-256. [doi: annurev.ge.05.120171.001321]

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1. The paper by Kellis et al. (2014) is ambiguous. It's clear that most of the ENCODE authors are still opposed to junk DNA even though the paper is mostly a retraction of their original claim that 80% of the genome is functional.

Subhash Lakhotia: The concept of 'junk DNA' becomes junk

Continuing my survey of recent papers on junk DNA, I stumbled upon a review by Subash Lakhotia that has recently been accepted in The Proceedings of the Indian National Science Academy (Lakhotia, 2018). It illustrates the extent of the publicity campaign mounted by ENCODE and opponents of junk DNA. In the title of this post, I paraphrased a sentence from the abstract that summarizes the point of the paper; namely, that the 'recent' discovery of noncoding RNAs refutes the concept of junk DNA.

Lakhotia claims to have written a review of the history of junk DNA but, in fact, his review perpetuates a false history. He repeats a version of history made popular by John Mattick. It goes like this. Old-fashioned scientists were seduced by Crick's central dogma into thinking that the only important part of the genome was the part encoding proteins. They ignored genes for noncoding RNAs because they didn't fit into their 'dogma.' They assumed that most of the noncoding part of the genome was junk. However, recent new discoveries of huge numbers of noncoding RNAs reveal that those scientists were very stupid. We now know that the genome is chock full of noncoding RNA genes and the concept of junk DNA has been refuted.

Peter Larsen: "There is no such thing as 'junk DNA'"

The March 2018 issue of Chromosome Research is a Special Issue on Transposable Elements and Genome Function. I found it as I was doing my routine search for papers on junk DNA in order to see whether scientists are finally beginning to understand the issue. Peter Larsen (guest editor) wrote the introduction to the special issue. He says ...

There is no such thing as “junk DNA.” Indeed, a suite of discoveries made over the past few decades have put to rest this misnomer and have identified many important roles that so-called junk DNA provides to both genome structure and function (this special issue; Biémont and Vieira 2006; Jeck et al. 2013; Elbarbary et al. 2016; Akera et al. 2017; Chen and Yang 2017; Chuong et al. 2017). Nevertheless, given the historical focus on coding regions of the genome, our understanding of the biological function of non-coding regions (e.g., repetitive DNA, transposable elements) remains somewhat limited, and therefore, all those enigmatic and poorly studied regions of the genome that were once identified as junk are instead best viewed as genomic “dark matter.”

What's In Your Genome? - The Pie Chart

Here's my latest compilation of the composition of the human genome. It's depicted in the form of a pie chart.¹ [UPDATED: March 29, 2018]

Making Sense of Genes by Kostas Kampourakis

Kostas Kampourakis is a specialist in science education at the University of Geneva, Geneva (Switzerland). Most of his book is an argument against genetic determinism in the style of Richard Lewontin. You should read this book if you are interested in that argument. The best way to describe the main thesis is to quote from the last chapter.

Here is the take-home message of this book: Genes were initially conceived as immaterial factors with heuristic values for research, but along the way they acquired a parallel identity as DNA segments. The two identities never converged completely, and therefore the best we can do so far is to think of genes as DNA segments that encode functional products. There are neither 'genes for' characters nor 'genes for' diseases. Genes do nothing on their own, but are important resources for our self-regulated organism. If we insist in asking what genes do, we can accept that they are implicated in the development of characters and disease, and that they account for variation in characters in particular populations. Beyond that, we should remember that genes are part of an interactive genome that we have just begun to understand, the study of which has various limitations. Genes are not our essences, they do not determine who we are, and they are not the explanation of who we are and what we do. Therefore we are not the prisoners of any genetic fate. This is what the present book has aimed to explain.

Are splice variants functional or noise?

This is a post about alternative splicing. I've avoided using that term in the title because it's very misleading. Alternative splicing produces a number of different products (RNA or protein) from a single intron-containing gene. The phenomenon has been known for 35 years and there are quite a few very well-studied examples, including several where all of the splice regulatory factors have been characterized.

How many lncRNAs are functional?

There's solid evidence that 90% of your genome is junk. Most of it is transcribed at some time but the transcripts are transient and usually confined to the nucleus. They are junk RNA [Functional RNAs?]. This is the view held by many experts but you wouldn't know that from reading the scientific literature and the popular press. The opposition to junk DNA gets much more attention in both venues.

There are prominent voices expressing the view that most of the genome is devoted to producing functional RNAs required for regulating gene expression [John Mattick still claims that most lncRNAs are functional]. Most of these RNAs are long noncoding RNAs known as lncRNAs. Although most of them fail all reasonable criteria for function there are still those who maintain that tens of thousands of them are functional [How many lncRNAs are functional: can sequence comparisons tell us the answer?].

ENCODE's false claims about the number of regulatory sites per gene

Some beating of dead horses may be ethical, where here and there they display unexpected twitches that look like life.

Zuckerkandl and Pauling (1965)

I realize that most of you are tired of seeing criticisms of ENCODE but it's important to realize that most scientists fell hook-line-and-sinker for the ENCODE publicity campaign and they still don't know that most of the claims were ridiculous.

I was reminded of this when I re-read Brendan Maher's summary of the ENCODE results that were published in Nature on Sept. 6, 2012 (Maher, 2012). Maher's article appeared in the front section of the ENCODE issue.¹ With respect to regulatory sequences he said ...

The consortium has assigned some sort of function to roughly 80% of the genome, including more than 70,000 ‘promoter’ regions — the sites, just upstream of genes, where proteins bind to control gene expression — and nearly 400,000 ‘enhancer’ regions that regulate expression of distant genes ... But the job is far from done, says [Ewan] Birney, a computational biologist at the European Molecular Biology Laboratory’s European Bioinformatics Institute in Hinxton, UK, who coordinated the data analysis for ENCODE. He says that some of the mapping efforts are about halfway to completion, and that deeper characterization of everything the genome is doing is probably only 10% finished.

What's in Your Genome?: Chapter 5: Regulation and Control of Gene Expression

I'm working (slowly) on a book called What's in Your Genome?: 90% of your genome is junk! The first chapter is an introduction to genomes and DNA [What's in Your Genome? Chapter 1: Introducing Genomes ]. Chapter 2 is an overview of the human genome. It's a summary of known functional sequences and known junk DNA [What's in Your Genome? Chapter 2: The Big Picture]. Chapter 3 defines "genes" and describes protein-coding genes and alternative splicing [What's in Your Genome? Chapter 3: What Is a Gene?]. Chapter 4 is all about pervasive transcription and genes for functional noncoding RNAs [What's in Your Genome? Chapter 4: Pervasive Transcription].

Chapter 5 is Regulation and Control of Gene Expression.

Chapter 5: Regulation and Control of Gene Expression

What do we know about regulatory sequences?

The fundamental principles of regulation were worked out in the 1960s and 1970s by studying bacteria and bacteriophage. The initiation of transcription is controlled by activators and repressors that bind to DNA near the 5′ end of a gene. These transcription factors recognize relatively short sequences of DNA (6-10 bp) and their interactions have been well-characterized. Transcriptional regulation in eukaryotes is more complicated for two reasons. First, there are usually more transcription factors and more binding sites per gene. Second, access to binding sites depends of the state of chromatin. Nucleosomes forming high order structures create a "closed" domain where DNA binding sites are not accessible. In "open" domains the DNA is more accessible and transcription factors can bind. The transition between open and closed domains is an important addition to regulating gene expression in eukaryotes.

The limitations of genomics

By their very nature, genomics studies look at the big picture. Such studies can tell us a lot about how many transcription factors bind to DNA and how much of the genome is transcribed. They cannot tell you whether the data actually reflects function. For that, you have to take a more reductionist approach and dissect the roles of individual factors on individual genes. But working on single genes can be misleading ... you may miss the forest for the trees. Genomic studies have the opposite problem, they may see a forest where there are no trees.

Regulation and evolution

Much of what we see in evolution, especially when it comes to phenotypic differences between species, is due to differences in the regulation of shared genes. The idea dates back to the 1930s and the mechanisms were worked out mostly in the 1980s. It's the reason why all complex animals should have roughly the same number of genes—a prediction that was confirmed by sequencing the human genome. This is the field known as evo-devo or evolutionary developmental biology.

           Box 5-1: Can complex evolution evolve by accident?

Slightly harmful mutations can become fixed in a small population. This may cause a gene to be transcribed less frequently. Subsequent mutations that restore transcription may involve the binding of an additional factor to enhance transcription initiation. The result is more complex regulation that wasn't directly selected.

Open and closed chromatin domains

Gene expression in eukaryotes is regulated, in part, by changing the structure of chromatin. Genes in domains where nucleosomes are densely packed into compact structures are essentially invisible. Genes in more open domains are easily transcribed. In some species, the shift between open and closed domains is associated with methylation of DNA and modifications of histones but it's not clear whether these associations cause the shift or are merely a consequence of the shift.

           Box 5-2: X-chromosome inactivation

In females, one of the X-chromosomes is preferentially converted to a heterochromatic state where most of the genes are in closed domains. Consequently, many of the genes on the X chromosome are only expressed from one copy as is the case in males. The partial inactivation of an X-chromosome is mediated by a small regulatory RNA molecule and this inactivated state is passed on to all subsequent descendants of the original cell.

           Box 5-3: Regulating gene expression by
           rearranging the genome

In several cases, the regulation of gene expression is controlled by rearranging the genome to bring a gene under the control of a new promoter region. Such rearrangements also explain some developmental anomalies such as growth of legs on the head fruit flies instead of antennae. They also account for many cancers.

ENCODE does it again

Genomic studies carried out by the ENCODE Consortium reported that a large percentage of the human genome is devoted to regulation. What the studies actually showed is that there are a large number of binding sites for transcription factors. ENCODE did not present good evidence that these sites were functional.

Does regulation explain junk?

The presence of huge numbers of spurious DNA binding sites is perfectly consistent with the view that 90% of our genome is junk. The idea that a large percentage of our genome is devoted to transcriptional regulation is inconsistent with everything we know from the the studies of individual genes.

           Box 5-3: A thought experiment

Ford Doolittle asks us to imagine the following thought experiment. Take the fugu genome, which is very much smaller than the human genome, and the lungfish genome, which is very much larger, and subject them to the same ENCODE analysis that was performed on the human genome. All three genomes have approximately the same number of genes and most of those genes are homologous. Will the number of transcription factor biding sites be similar in all three species or will the number correlate with the size of the genomes and the amount of junk DNA?

Small RNAs—a revolutionary discovery?

Does the human genome contain hundreds of thousands of gene for small non-coding RNAs that are required for the complex regulation of the protein-coding genes?

A “theory” that just won’t die

"... we have refuted the specific claims that most of the observed transcription across the human genome is random and put forward the case over many years that the appearance of a vast layer of RNA-based epigenetic regulation was a necessary prerequisite to the emergence of developmentally and cognitively advanced organisms." (Mattick and Dinger, 2013)

What the heck is epigenetics?

Epigenetics is a confusing term. It refers loosely to the regulation of gene expression by factors other than differences in the DNA. It's generally assumed to cover things like methylation of DNA and modification of histones. Both of these effects can be passed on from one cell to the next following mitosis. That fact has been known for decades. It is not controversial. The controversy is about whether the heritability of epigenetic features plays a significant role in evolution.

           Box 5-5: The Weismann barrier

The Weisman barrier refers to the separation between somatic cells and the germ line in complex multicellular organisms. The "barrier" is the idea that changes (e.g. methylation, histone modification) that occur in somatic cells can be passed on to other somatic cells but in order to affect evolution those changes have to be transferred to the germ line. That's unlikely. It means that Lamarckian evolution is highly improbable in such species.

How should science journalists cover this story?

The question is whether a large part of the human genome is devoted to regulation thus accounting for an unexpectedly large genome. It's an explanation that attempts to refute the evidence for junk DNA. The issue is complex and very few science journalists are sufficiently informed enough to do it justice. They should, however, be making more of an effort to inform themselves about the controversial nature of the claims made by some scientists and they should be telling their readers that the issue has not yet been resolved.

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How much mitochondrial DNA in your genome?

Most mitochondrial genes have been transferred from the ancestral mitochondrial genome to the nuclear genome over the course of 1-2 billion years of evollution. They are no longer present in mitochondria but they are easily recognized because they resemble α-proteobacterial sequences more than the other nuclear genes [see Endosymbiotic Theory].

This process of incorporating mitochondrial DNA into the nuclear genome continues to this day. The latest human reference genome has about 600 examples of nuclear sequences of mitochondrial origin (= numts). Some of them are quite recent while others date back almost 70 million years—the limit of resolution for junk DNA [see Mitochondria are invading your genome!].

Creationists questioning pseudogenes: the GULO pseudogene

This is the second post discussing creationist¹ papers on pseudogenes. The first post addressed a paper by Jeffrey Tomkins on the β-globin pseudogene [Creationists questioning pseudogenes: the beta-globin pseudogene]. This post covers another paper by Tomkins claiming that the GULO pseudogenes in various primate species are not derived from a common ancestor but instead have been deactivated independently in each lineage.

The Tomkins' article was published in 2014 in Answers Research Journal, a publication that describes itself like this:

ARJ is a professional, peer-reviewed technical journal for the publication of interdisciplinary scientific and other relevant research from the perspective of the recent Creation and the global Flood within a biblical framework.

Creationists questioning pseudogenes: the beta-globin pseudogene

Jonathan Kane recently (Oct. 6, 2017) posted an article on The Panda's Thumb where he claimed that Young Earth Creationists often don't get enough credit for raising serious issues about evolution [Five principles for arguing against creationism].

He mentioned some articles about pseudogenes as prime examples. I asked him for references and he responded with two articles by Jeffrey Tomkins that were published on the Answers in Genesis website. The first was on the β-globin pseudogene and the second was on the GULO pseudogene. Both articles claim that these DNA sequences aren't really pseudogenes because they have functions.

I'll deal with the β-globin pseudogene in this post and the GULO pseudogene in a subsequent post.

What's in Your Genome?: Chapter 4: Pervasive Transcription (revised)

I'm working (slowly) on a book called What's in Your Genome?: 90% of your genome is junk! The first chapter is an introduction to genomes and DNA [What's in Your Genome? Chapter 1: Introducing Genomes ]. Chapter 2 is an overview of the human genome. It's a summary of known functional sequences and known junk DNA [What's in Your Genome? Chapter 2: The Big Picture]. Chapter 3 defines "genes" and describes protein-coding genes and alternative splicing [What's in Your Genome? Chapter 3: What Is a Gene?].

Chapter 4 is all about pervasive transcription and genes for functional noncoding RNAs. I've finally got a respectable draft of this chapter. This is an updated summary—the first version is at: What's in Your Genome? Chapter 4: Pervasive Transcription.

Cold Spring Harbor tells us about the "dark matter" of the genome (Part I)

This is a podcast from Cold Spring Harbor [Dark Matter of the Genome, Pt. 1 (Base Pairs Episode 8)]. The authors try to convince us that most of the genome is mysterious "dark matter," not junk. The main theme is that the genome contains transposons that could play an important role in evolution and disease.

Experts meet to discuss non-coding RNAs - fail to answer the important question

The human genome is pervasively transcribed. More than 80% of the genome is complementary to transcripts that have been detected in some tissue or cell type. The important question is whether most of these transcripts have a biological function. How many genes are there that produce functional non-coding RNA?

There's a reason why this question is important. It's because we have every reason to believe that spurious transcription is common in large genomes like ours. Spurious, or accidental, transcription occurs when the transcription initiation complex binds nonspecifically to sites in the genome that are not real promoters. Spurious transcription also occurs when the initiation complex (RNA plymerase plus factors) fires in the wrong direction from real promoters. Binding and inappropriate transcription are aided by the binding of transcription factors to nonpromoter regions of the genome—a well-known feature of all DNA binding proteins [see Are most transcription factor binding sites functional?].

How much of the human genome is devoted to regulation?

All available evidence suggests that about 90% of our genome is junk DNA. Many scientists are reluctant to accept this evidence—some of them are even unaware of the evidence [Five Things You Should Know if You Want to Participate in the Junk DNA Debate]. Many opponents of junk DNA suffer from what I call The Deflated Ego Problem. They are reluctant to concede that humans have about the same number of genes as all other mammals and only a few more than insects.

One of the common rationalizations is to speculate that while humans may have "only" 25,000 genes they are regulated and controlled in a much more sophisticated manner than the genes in other species. It's this extra level of control that makes humans special. Such speculations have been around for almost fifty years but they have gained in popularity since publication of the human genome sequence.

In some cases, the extra level of regulation is thought to be due to abundant regulatory RNAs. This means there must be tens of thousand of extra genes expressing these regulatory RNAs. John Mattick is the most vocal proponent of this idea and he won an award from the Human Genome Organization for "proving" that his speculation is correct! [John Mattick Wins Chen Award for Distinguished Academic Achievement in Human Genetic and Genomic Research]. Knowledgeable scientists know that Mattick is probably wrong. They believe that most of those transcripts are junk RNAs produced by accidental transcription at very low levels from non-conserved sequences.

Revisiting the genetic load argument with Dan Graur

The genetic load argument is one of the oldest arguments for junk DNA and it's one of the most powerful arguments that most of our genome must be junk. The concept dates back to J.B.S. Haldane in the late 1930s but the modern argument traditionally begins with Hermann Muller's classic paper from 1950. It has been extended and refined by him and many others since then (Muller, 1950; Muller, 1966).

Debating alternative splicing (part II)

Mammalian genomes are very large. It looks like 90% of it is junk DNA. These genomes are pervasively transcribed, meaning that almost 90% of the bases are complementary to a transcript produced at some time during development. I think most of those transcripts are due to inappropriate transcription initiation. They are mistakes in transcription. The genome is littered with transcription factor binding sites but only a small percentage are directly involved in regulating gene expression. The rest are due to spurious binding—a well-known property of DNA binding proteins. These conclusions are based, I believe, on a proper understanding of evolution and basic biochemistry.

If you add up all the known genes, they cover about 30% of the genome sequence. Most of this (>90%) is intron sequence and introns are mostly junk. The standard mammalian gene is transcribed to produce a precursor RNA that is subsequently processed by splicing out introns to produce a mature RNA. If it's a messenger RNA (mRNA) then it will be translated to produce a protein (technically, a polypeptide). So far, the vast majority of protein-coding genes produce a single protein but there are some classic cases of alternative splicing where a given gene produces several different protein isoforms, each of which has a specific function.