Abstract

Many organisms face a dilemma rooted in the unequal numbers of X chromosomes carried by the two sexes and the need to maintain equivalent expression of X‐linked genes. Several strategies have arisen to cope with this problem. All rely on accurately targeting epigenetic modifications to entire chromosomes. Targeting results from the action of recognition elements that attract modification and may rely on spreading of modification in cis along the affected chromosome. A recent report describing the first X chromosome recognition element from C. elegans opens the way to defining the relative contributions of these factors to the compensation of X‐linked gene expression in worms.1 Extrachromosomal arrays composed of a C. elegans recognition element attract proteins that modify the C. elegans X chromosomes and interact genetically with mutations disrupting compensation. Moreover, examination of X:A translocations provides the first evidence for spreading of modification along C. elegans X chromosomes. BioEssays 26:707–710, 2004. © 2004 Wiley Periodicals, Inc.