X-inactivation: Difference between revisions

[[Image:BarrBodyBMC Biology2-21-Fig1clip293px.jpg|thumb|An interphase female human fibroblast cell.<ref>{{cite journal | vauthors = Gartler SM, Varadarajan KR, Luo P, Canfield TK, Traynor J, Francke U, Hansen RS | title = Normal histone modifications on the inactive X chromosome in ICF and Rett syndrome cells: implications for methyl-CpG binding proteins | journal = BMC Biology | volume = 2 | pages = 21 | date = September 2004 | pmid = 15377381 | pmc = 521681 | doi = 10.1186/1741-7007-2-21 | doi-access = free }}</ref> Arrows point to sex chromatin on DNA ([[DAPI]]) in cell nucleus(left), and to the corresponding X chromatin (right).<br />Left: DNA (DAPI)-stained nucleus. Arrow indicates the location of Barr body(Xi). Right: DNA associated [[histone]]s protein detected]]

'''X-inactivation''' (also called '''Lyonization''', after English geneticist [[Mary F. Lyon|Mary Lyon]]) is a process by which one of the copies of the [[X chromosome]] is inactivated in [[theria]]n [[female]] [[mammal]]s. The inactive X chromosome is silenced by being packaged into a transcriptionally inactive structure called [[heterochromatin]]. As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome [[gene product]]s as [[male]]smales, who only possess a single copy of the X chromosome (see [[dosage compensation]]).

The choice of which X chromosome will be inactivated in a particular embryonic cell is random in [[eutheria|placental mammals]] such as [[human]]shumans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism (its cell line). The result is that the choice of inactivated X chromosome in all the cells of the organism is a random distribution, often with about half the cells having the paternal X chromosome inactivated and half with an inactivated maternal X chromosome; but commonly, X-inactivation is unevenly distributed across the cell lines within one organism ([[skewed X-inactivation]]).

X-inactivation is part of the activation cycle of the X chromosome throughout the female life. The egg and the fertilized zygote initially use maternal transcripts, and the whole embryonic genome is silenced until [[zygotic genome activation]]. Thereafter, all mouse cells undergo an early, [[Imprinting (genetics)|imprinted]] inactivation of the paternally-derived X chromosome in [[mammalian embryogenesis|4–8 cell stage]] [[embryo]]s.<ref>{{cite journal | vauthors = Takagi N, Sasaki M | title = Preferential inactivation of the paternally derived X chromosome in the extra embryonic membranes of the mouse | journal = Nature | volume = 256 | issue = 5519 | pages = 640–2 | date = August 1975 | pmid = 1152998 | doi = 10.1038/256640a0 | bibcode = 1975Natur.256..640T | s2cid = 4190616 }}</ref><ref>{{cite journal | vauthors = Cheng MK, Disteche CM | title = Silence of the fathers: early X inactivation | journal = BioEssays | volume = 26 | issue = 8 | pages = 821–4 | date = August 2004 | pmid = 15273983 | doi = 10.1002/bies.20082 | url = http://www3.interscience.wiley.com/cgi-bin/fulltext/109565168/PDFSTART | doi-access = free }}{{dead link|date=February 2019|bot=medic}}{{cbignore|bot=medic}}</ref><ref name="okamoto">{{cite journal | vauthors = Okamoto I, Otte AP, Allis CD, Reinberg D, Heard E | title = Epigenetic dynamics of imprinted X inactivation during early mouse development | journal = Science | volume = 303 | issue = 5658 | pages = 644–9 | date = January 2004 | pmid = 14671313 | doi = 10.1126/science.1092727 | bibcode = 2004Sci...303..644O | s2cid = 26326026 }}</ref><ref name=":2">{{cite journal | vauthors = Deng Q, Ramsköld D, Reinius B, Sandberg R | title = Single-cell RNA-seq reveals dynamic, random monoallelic gene expression in mammalian cells | journal = Science | volume = 343 | issue = 6167 | pages = 193–6 | date = January 2014 | pmid = 24408435 | doi = 10.1126/science.1245316 | bibcode = 2014Sci...343..193D | s2cid = 206552108 }}</ref> The [[extraembryonic tissue]]s (which give rise to the [[placenta]] and other tissues supporting the embryo) retain this early imprinted inactivation, and thus only the maternal X chromosome is active in these tissues.

In the early [[blastocyst]], this initial, imprinted X-inactivation is [[X-chromosome reactivation|reversed]] in the cells of the [[inner cell mass]] (which give rise to the embryo), and in these cells both X chromosomes become active again. Each of these cells then independently and randomly inactivates one copy of the X chromosome.<ref name=okamoto/> This inactivation event is irreversible during the lifetime of the individual, with the exception of the germline. In the female [[germline]] before meiotic entry, X-inactivation is reversed, so that after meiosis all haploid [[oocyte]]s contain a single active X chromosome.

# Xi^P Xi^M zygote → undergoing [[Maternal to zygotic transition|zygotic genome activation]], leading to:

The descendants of each cell which inactivated a particular X chromosome will also inactivate that same chromosome. This phenomenon, which can be observed in the coloration of [[tortoiseshell cat]]s when females are [[heterozygous]] for the [[sex linkage|X-linked gene]] pigment gene, should not be confused with [[mosaic (genetics)|mosaicism]], which is a term that specifically refers to differences in the [[genotype]] of various cell populations in the same individual; X-inactivation, which is an [[epigenetics|epigenetic]] change that results in a different phenotype, is ''not'' a change at the [[genotype|genotypic]] level. For an individual cell or lineage the inactivation is therefore [[Skewed X-inactivation|skewed]] or '[[Skewed X-inactivation|non-random]]', and this can give rise to mild symptoms in female 'carriers' of [[X-linked]] genetic disorders.<ref>{{cite journal | vauthors = Puck JM, Willard HF | title = X inactivation in females with X-linked disease | journal = The New England Journal of Medicine | volume = 338 | issue = 5 | pages = 325–8 | date = January 1998 | pmid = 9445416 | doi = 10.1056/NEJM199801293380611 }}</ref>

It is understood that X-chromosome inactivation is a random process, occurring at about the time of [[gastrulation]] in the [[epiblast]] (cells that will give rise to the embryo). The maternal and paternal X chromosomes have an equal probability of inactivation. This would suggest that women would be expected to suffer from X-linked disorders approximately 50% as often as men (because women have two X chromosomes, while men have only one); however, in actuality, the occurrence of these disorders in females is much lower than that. One explanation for this disparity is that 12–20% <ref>{{cite journal | vauthors = Balaton BP, Cotton AM, Brown CJ | title = Derivation of consensus inactivation status for X-linked genes from genome-wide studies | journal = Biology of Sex Differences | volume = 6 | issue = 35 | pages = 35 | date = 30 December 2015 | pmid = 26719789 | pmc = 4696107 | doi = 10.1186/s13293-015-0053-7 | doi-access = free }}</ref> of genes on the inactivated X chromosome remain expressed, thus providing women with added protection against defective genes coded by the X-chromosome. Some{{who|date=October 2014}} suggest that this disparity must be evidence of preferential (non-random) inactivation. Preferential inactivation of the paternal X-chromosome occurs in both marsupials and in cell lineages that form the membranes surrounding the embryo,<ref>{{cite journal | vauthors = Graves JA | title = Mammals that break the rules: genetics of marsupials and monotremes | journal = Annual Review of Genetics | volume = 30 | pages = 233–60 | year = 1996 | pmid = 8982455 | doi = 10.1146/annurev.genet.30.1.233 }}</ref> whereas in placental mammals either the maternally or the paternally derived X-chromosome may be inactivated in different cell lines.<ref>{{cite journal | vauthors = Lyon MF | title = X-chromosome inactivation and developmental patterns in mammals | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 47 | issue = 1 | pages = 1–35 | date = January 1972 | pmid = 4554151 | doi = 10.1111/j.1469-185X.1972.tb00969.x | s2cid = 39402646 }}</ref>

The time period for X-chromosome inactivation explains this disparity. Inactivation occurs in the epiblast during gastrulation, which gives rise to the embryo.<ref>{{cite journal| vauthors = Migeon, B |title=X chromosome inactivation in human cells|url=http://hstalks.com/main/view_talk.php?t=1676&r=494&c=252|journal=The Biomedical & Life Sciences Collection|publisher=Henry Stewart Talks, Ltd|access-date=15 December 2013 |pages=1–54|year=2010}}</ref> Inactivation occurs on a cellular level, resulting in a mosaic expression, in which patches of cells have an inactive maternal X-chromosome, while other patches have an inactive paternal X-chromosome. For example, a female heterozygous for haemophilia (an X-linked disease) would have about half of her liver cells functioning properly, which is typically enough to ensure normal blood clotting.<ref name="Gartler_2001">{{cite journal | last1 = Gartler | first1 = Stanley M | last2 = Goldman | first2 = Michael A | name-list-style = vanc | title = X-Chromosome Inactivation | url = http://web.udl.es/usuaris/e4650869/docencia/segoncicle/genclin98/recursos_classe_(pdf)/revisionsPDF/XChromoInac.pdf | journal = Encyclopedia of Life Sciences | publisher = Nature Publishing Group | pages = 1–2 | year=2001 }}</ref><ref name="Connallon_2013">{{cite journal | vauthors = Connallon T, Clark AG | title = Sex-differential selection and the evolution of X inactivation strategies | journal = PLOS Genetics | volume = 9 | issue = 4 | pages = e1003440 | date = April 2013 | pmid = 23637618 | pmc = 3630082 | doi = 10.1371/journal.pgen.1003440 | doi-access = free }}</ref> Chance could result in significantly more dysfunctional cells; however, such statistical extremes are unlikely. Genetic differences on the chromosome may also render one X-chromosome more likely to undergo inactivation. Also, if one X-chromosome has a mutation hindering its growth or rendering it non viable, cells which randomly inactivated that X will have a selective advantage over cells which randomly inactivated the normal allele. Thus, although inactivation is initially random, cells that inactivate a normal allele (leaving the mutated allele active) will eventually be overgrown and replaced by functionally normal cells in which nearly all have the same X-chromosome activated.<ref name="Gartler_2001"/>

It is hypothesized that there is an autosomally-encoded 'blocking factor' which binds to the X chromosome and prevents its inactivation.<ref>{{Cite journal |last1=Avner |first1=Philip |last2=Heard |first2=Edith |date=January 2001 |title=X-chromosome inactivation: counting, choice and initiation |url=https://www.nature.com/articles/35047580 |journal=Nature Reviews Genetics |language=en |volume=2 |issue=1 |pages=59–67 |doi=10.1038/35047580 |pmid=11253071 |s2cid=5234164 |issn=1471-0064}}</ref> that there is an autosomally-encoded 'blocking factor' which binds to the X chromosome and prevents its inactivation. The model postulates that there is a limiting blocking factor, so once the available blocking factor molecule binds to one X chromosome the remaining X chromosome(s) are not protected from inactivation. This model is supported by the existence of a single Xa in cells with many X chromosomes and by the existence of two active X chromosomes in cell lines with twice the normal number of autosomes.<ref>{{cite book | first1 = Tahsin Stefan | last1 = Barakat | first2 = Joost | last2 = Gribnau | name-list-style = vanc | chapter = X Chromosome Inactivation and Embryonic Stem Cells | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK45037/ | title = The Cell Biology of Stem Cells | editor-first1 = Eran | editor-last1 = Meshorer | editor-first2 = Kathrin | editor-last2 = Plath | year = 2010 | publisher = Landes Bioscience and Springer Science+Business Media }}</ref>

Since males only have one copy of the X chromosome, all expressed X-chromosomal [[gene]]s (or [[allele]]s, in the case of multiple variant forms for a given gene in the population) are located on that copy of the chromosome. Females, however, will primarily express the genes or alleles located on the X-chromosomal copy that remains active. Considering the situation for one gene or multiple genes causing individual differences in a particular [[Phenotypic trait|phenotype]] (i.e., causing variation observed in the population for that phenotype), in homozygous females it doesn'tdoes not particularly matter which copy of the chromosome is inactivated, as the alleles on both copies are the same. However, in females that are heterozygous at the causal genes, the inactivation of one copy of the chromosome over the other can have a direct impact on their phenotypic value. Because of this phenomenon, there is an observed increase in phenotypic variation in females that are heterozygous at the involved gene or genes than in females that are homozygous at that gene or those genes.<ref>{{cite journal | vauthors = Ma L, Hoffman G, Keinan A | title = X-inactivation informs variance-based testing for X-linked association of a quantitative trait | journal = BMC Genomics | volume = 16 | pages = 241 | date = March 2015 | issue = 1 | pmid = 25880738 | pmc = 4381508 | doi = 10.1186/s12864-015-1463-y | doi-access = free }}</ref> There are many different ways in which the phenotypic variation can play out. In many cases, heterozygous females may be asymptomatic or only present minor symptoms of a given disorder, such as with [[Adrenoleukodystrophy|X-linked adrenoleukodystrophy.]]<ref>{{cite journal | vauthors = Habekost CT, Pereira FS, Vargas CR, Coelho DM, Torrez V, Oses JP, Portela LV, Schestatsky P, Felix VT, Matte U, Torman VL, Jardim LB | title = Progression rate of myelopathy in X-linked adrenoleukodystrophy heterozygotes | journal = Metabolic Brain Disease | volume = 30 | issue = 5 | pages = 1279–84 | date = October 2015 | pmid = 25920484 | doi = 10.1007/s11011-015-9672-2 | s2cid = 11375978 }}</ref>

On average, each X chromosome is inactivated in half of the cells, howeveralthough 5-20% of "apparently normal" women display X-inactivation skewing.<ref name=":0" /> In cases where skewing is present, a broad range of symptom expression can occur, resulting in expression varying from minor to severe depending on the skewing proportion. An extreme case of this was seen where monozygotic female twins had extreme variance in expression of [[Menkes disease]] (an X-linked disorder) resulting in the death of one twin while the other remained asymptomatic.<ref>{{cite journal | vauthors = Burgemeister AL, Zirn B, Oeffner F, Kaler SG, Lemm G, Rossier E, Büttel HM | title = Menkes disease with discordant phenotype in female monozygotic twins | journal = American Journal of Medical Genetics. Part A | volume = 167A | issue = 11 | pages = 2826–9 | date = November 2015 | pmid = 26239182 | pmc = 6475897 | doi = 10.1002/ajmg.a.37276 }}</ref>

It is thought that X-inactivation skewing could be caused by issues in the mechanism that causes inactivation, or by issues in the chromosome itself.<ref name=":0" /><ref name=":1" /> However, the link between phenotype and skewing is still being questioned, and should be examined on a case-by-case basis. A study looking at both symptomatic and asymptomatic females who were heterozygous for [[Duchenne muscular dystrophy|Duchenne]] and Becker muscular dystrophies (DMD) found no apparent link between transcript expression and skewed X-Inactivation. The study suggests that both mechanisms are independently regulated, and there are other unknown factors at play.<ref name="pmid22894145">{{cite journal | vauthors = Brioschi S, Gualandi F, Scotton C, Armaroli A, Bovolenta M, Falzarano MS, Sabatelli P, Selvatici R, D'Amico A, Pane M, Ricci G, Siciliano G, Tedeschi S, Pini A, Vercelli L, De Grandis D, Mercuri E, Bertini E, Merlini L, Mongini T, Ferlini A | title = Genetic characterization in symptomatic female DMD carriers: lack of relationship between X-inactivation, transcriptional DMD allele balancing and phenotype | journal = BMC Medical Genetics | volume = 13 | pages = 73 | date = August 2012 | pmid = 22894145 | pmc = 3459813 | doi = 10.1186/1471-2350-13-73 | doi-access = free }}</ref>

The X-inactive specific transcript ([[Xist]]) gene encodes a large [[NcRNA|non-coding RNA]] that is responsible for mediating the specific silencing of the X chromosome from which it is transcribed.<ref>{{cite journal | vauthors = Hoki Y, Kimura N, Kanbayashi M, Amakawa Y, Ohhata T, Sasaki H, Sado T | title = A proximal conserved repeat in the Xist gene is essential as a genomic element for X-inactivation in mouse | journal = Development | volume = 136 | issue = 1 | pages = 139–46 | date = January 2009 | pmid = 19036803 | doi = 10.1242/dev.026427 | doi-access = free }}</ref> The inactive X chromosome is coated by Xist RNA,<ref>{{cite journal | vauthors = Ng K, Pullirsch D, Leeb M, Wutz A | title = Xist and the order of silencing | journal = EMBO Reports | volume = 8 | issue = 1 | pages = 34–9 | date = January 2007 | pmid = 17203100 | pmc = 1796754 | doi = 10.1038/sj.embor.7400871 | format = Review Article | quote = [httphttps://www.pubmedcentralncbi.nlm.nih.gov/articlerender.fcgi?artid=1796754&rendertype=figure&id=f1pmc/articles/PMC1796754/ Figure 1 Xist RNA encompasses the X from which it is transcribed.] }}</ref> whereas the Xa is not (See Figure to the right). X chromosomes that lack the Xist gene cannot be inactivated.<ref>{{cite journal | vauthors = Penny GD, Kay GF, Sheardown SA, Rastan S, [[Neil Brockdorff|Brockdorff N]] | title = Requirement for Xist in X chromosome inactivation | journal = Nature | volume = 379 | issue = 6561 | pages = 131–7 | year = 1996 | pmid = 8538762 | doi = 10.1038/379131a0 | bibcode = 1996Natur.379..131P | s2cid = 4329368 }}</ref> Artificially placing and expressing the Xist gene on another chromosome leads to silencing of that chromosome.<ref name="Herzing">{{cite journal | vauthors = Herzing LB, Romer JT, Horn JM, Ashworth A | title = Xist has properties of the X-chromosome inactivation centre | journal = Nature | volume = 386 | issue = 6622 | pages = 272–5 | date = March 1997 | pmid = 9069284 | doi = 10.1038/386272a0 | bibcode = 1997Natur.386..272H | s2cid = 4371247 }}</ref><ref name=":6">{{cite journal | vauthors = Lee JT, Jaenisch R | title = Long-range cis effects of ectopic X-inactivation centres on a mouse autosome | journal = Nature | volume = 386 | issue = 6622 | pages = 275–9 | date = March 1997 | pmid = 9069285 | doi = 10.1038/386275a0 | bibcode = 1997Natur.386..275L | s2cid = 10899129 }}</ref>

Compared to the Xa, the Xi has high levels of [[DNA methylation]], low levels of [[histone acetylation]], low levels of [[histone H3]] lysine-4 [[histone methylation|methylation]], and high levels of histone H3 lysine-9 methylation and H3 lysine-27 methylation mark which is placed by the [[Polycomb recruitment in X chromosome inactivation|PRC2 complex recruited by Xist]], all of which are associated with gene silencing.<ref>{{cite journal | vauthors = Ng K, Pullirsch D, Leeb M, Wutz A | title = Xist and the order of silencing | journal = EMBO Reports | volume = 8 | issue = 1 | pages = 34–9 | date = January 2007 | pmid = 17203100 | pmc = 1796754 | doi = 10.1038/sj.embor.7400871 | format = Review Article | quote = [httphttps://www.pubmedcentralncbi.nlm.nih.gov/articlerender.fcgi?artid=1796754&rendertype=table&id=t1pmc/articles/PMC1796754/ Table 1 Features of the inactive X territory] }} – Originated from;<br />{{cite journal | vauthors = Chow JC, Yen Z, Ziesche SM, Brown CJ | title = Silencing of the mammalian X chromosome | journal = Annual Review of Genomics and Human Genetics | volume = 6 | pages = 69–92 | year = 2005 | pmid = 16124854 | doi = 10.1146/annurev.genom.6.080604.162350 }}<br />{{cite journal | vauthors = Lucchesi JC, Kelly WG, Panning B | title = Chromatin remodeling in dosage compensation | journal = Annual Review of Genetics | volume = 39 | pages = 615–51 | year = 2005 | pmid = 16285873 | doi = 10.1146/annurev.genet.39.073003.094210 | citeseerx = 10.1.1.328.2992 }}</ref> [[PRC2]] regulates [[chromatin]] compaction and [[chromatin remodeling]] in several processes including the [[DNA damage (naturally occurring)|DNA damage]] response.<ref name="pmid28758948">{{cite journal |vauthors=Veneti Z, Gkouskou KK, Eliopoulos AG |title=Polycomb Repressor Complex 2 in Genomic Instability and Cancer |journal=Int J Mol Sci |volume=18 |issue=8 |pages= 1657|date=July 2017 |pmid=28758948 |pmc=5578047 |doi=10.3390/ijms18081657 |doi-access=free }}</ref> Additionally, a histone variant called macroH2A ([[H2AFY]]) is exclusively found on [[nucleosome]]s along the Xi.<ref name="Costanzi1998">{{cite journal | vauthors = Costanzi C, Pehrson JR | title = Histone macroH2A1 is concentrated in the inactive X chromosome of female mammals | journal = Nature | volume = 393 | issue = 6685 | pages = 599–601 | date = June 1998 | pmid = 9634239 | doi = 10.1038/31275 | bibcode = 1998Natur.393..599C | s2cid = 205001095 }}</ref><ref name="Costanzi2000">{{cite journal | vauthors = Costanzi C, Stein P, Worrad DM, Schultz RM, Pehrson JR | title = Histone macroH2A1 is concentrated in the inactive X chromosome of female preimplantation mouse embryos | journal = Development | volume = 127 | issue = 11 | pages = 2283–9 | date = June 2000 | doi = 10.1242/dev.127.11.2283 | pmid = 10804171 | url = http://dev.biologists.org/cgi/reprint/127/11/2283.pdf }}</ref>

The existence of genes along the inactive X which are not silenced explains the defects in humans with abnormalatypical numbers of the X chromosome, such as [[Turner syndrome]] (X0, caused by SHOX gene<ref>{{Cite web |title=Turner syndrome: MedlinePlus Genetics |url=https://medlineplus.gov/genetics/condition/turner-syndrome/ |access-date=2023-02-10 February 2023 |website=medlineplus.gov |language=en}}</ref>) or [[Klinefelter syndrome]] (XXY). Theoretically, X-inactivation should eliminate the differences in gene dosage between affected individuals and individuals with a normaltypical chromosome complement. In affected individuals, however, X-inactivation is incomplete and the dosage of these non-silenced genes will differ as they escape X-inactivation, similar to an autosomal [[aneuploidy]].

The precise mechanisms that control escape from X-inactivation are not known, but silenced and escape regions have been shown to have distinct chromatin marks.<ref name="Calabrese JM, Sun W, Song L, Mugford JW, Williams L, Yee D, Starmer J, Mieczkowski P, Crawford GE, Magnuson T 2012 951–63"/><ref>{{cite journal | vauthors = Berletch JB, Yang F, Disteche CM | title = Escape from X inactivation in mice and humans | journal = Genome Biology | volume = 11 | issue = 6 | pages = 213 | date = June 2010 | pmid = 20573260 | pmc = 2911101 | doi = 10.1186/gb-2010-11-6-213 | doi-access = free }}</ref> It has been suggested that escape from X-inactivation might be mediated by expression of [[long non-coding RNA]] (lncRNA) within the escaping chromosomal domains.<ref name="pmid21047393">{{cite journal | vauthors = Reinius B, Shi C, Hengshuo L, Sandhu KS, Radomska KJ, Rosen GD, Lu L, Kullander K, Williams RW, Jazin E | title = Female-biased expression of long non-coding RNAs in domains that escape X-inactivation in mouse | journal = BMC Genomics | volume = 11 | pages = 614 | date = November 2010 | pmid = 21047393 | pmc = 3091755 | doi = 10.1186/1471-2164-11-614 | doi-access = free }}</ref>

Besides, measuring the methylation (inactivation) status of the polymorphic human androgen receptor (HUMARA) located on X-chromosome is considered the most accurate method to assess clonality in female cancer biopsies.<ref>{{cite journal | vauthors = Chen GL, Prchal JT | title = X-linked clonality testing: interpretation and limitations | journal = Blood | volume = 110 | issue = 5 | pages = 1411–9 | date = September 2007 | pmid = 17435115 | pmc = 1975831 | doi = 10.1182/blood-2006-09-018655 }}</ref> A great variety of tumors was tested by this method, some, such as renal cell carcinoma,<ref>{{cite journal | vauthors = Petersson F, Branzovsky J, Martinek P, Korabecna M, Kruslin B, Hora M, Peckova K, Bauleth K, Pivovarcikova K, Michal M, Svajdler M, Sperga M, Bulimbasic S, Leroy X, Rychly B, Trivunic S, Kokoskova B, Rotterova P, Podhola M, Suster S, Hes O | display-authors = 6 | title = The leiomyomatous stroma in renal cell carcinomas is polyclonal and not part of the neoplastic process | journal = Virchows Archiv | volume = 465 | issue = 1 | pages = 89–96 | date = July 2014 | pmid = 24838683 | doi = 10.1007/s00428-014-1591-9 | s2cid = 24870232 }}</ref> found monoclonal while others (e.g. mesothelioma<ref>{{cite journal | vauthors = Comertpay S, Pastorino S, Tanji M, Mezzapelle R, Strianese O, Napolitano A, Baumann F, Weigel T, Friedberg J, Sugarbaker P, Krausz T, Wang E, Powers A, Gaudino G, Kanodia S, Pass HI, Parsons BL, Yang H, Carbone M | title = Evaluation of clonal origin of malignant mesothelioma | journal = Journal of Translational Medicine | volume = 12 | pages = 301 | date = December 2014 | pmid = 25471750 | pmc = 4255423 | doi = 10.1186/s12967-014-0301-3 | doi-access = free }}</ref>) were reported polyclonal.

* {{cite journal | vauthors = Cerase A, Pintacuda G, Tattermusch A, Avner P | title = Xist localization and function: new insights from multiple levels | journal = Genome Biology | volume = 16 | pages = 166 | date = August 2015 | issue = 1 | pmid = 26282267 | pmc = 4539689 | doi = 10.1186/s13059-015-0733-y | doi-access = free }}

* {{cite web | first = Karl | last = Kruszelnicki | name-list-style = vanc | url = http://www.abc.net.au/science/k2/moments/s1002754.htm | title = Hybrid Auto-Immune Women 3 | work = ABC Science | date = 2004-02-11 February 2004 }}