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X-chromosome inactivation is the mammalian dosage compensation mechanism by which transcription of X-linked genes is equalized between females and males. In an N-ethyl-N-nitrosourea (ENU) mutagenesis screen on mice for modifiers of epigenetic reprogramming, we identified the MommeD1 (modifier of murine metastable epialleles) mutation as a semidominant suppressor of variegation. MommeD1 shows homozygous female-specific mid-gestation lethality and hypomethylation of the X-linked gene Hprt1, suggestive of a defect in X inactivation. Here we report that the causative point mutation lies in a previously uncharacterized gene, Smchd1 (structural maintenance of chromosomes hinge domain containing 1). We find that SmcHD1 is not required for correct Xist expression, but localizes to the inactive X and has a role in the maintenance of X inactivation and the hypermethylation of CpG islands associated with the inactive X. This finding links a group of proteins normally associated with structural aspects of chromosome biology with epigenetic gene silencing.

Original publication




Journal article


Nat Genet

Publication Date





663 - 669


Animals, Chromosomal Proteins, Non-Histone, CpG Islands, DNA Methylation, Fibroblasts, Gene Silencing, Mice, Point Mutation, RNA, Long Noncoding, RNA, Untranslated, X Chromosome, X Chromosome Inactivation