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The Smcx gene is the first known example of a non-pseudoautosomal X-linked gene in mouse that normally escapes X chromosome inactivation. We have analysed the kinetics of escape at different stages of development, and in adult tissues. Our results demonstrate that Smcx exhibits partial escape from X inactivation in embryos, in extraembryonic lineages where paternally imprinted X inactivation occurs and also in adult tissues. The degree of escape in different tissues is highly variable, the level of transcript from the inactive X allele representing between 20% and 70% of the active X allele. Partial escape is also seen in clones derived from haematopoietic stem cells, suggesting that partial repression of the inactive X allele is at the level of individual cells. This contrasts with classical position effect variegation (PEV), where a given gene is either active or silent in a given cell and its clonal derivatives. We discuss the implications of these results with respect to mechanisms of X inactivation and escape.

Original publication




Journal article


Hum Mol Genet

Publication Date





1355 - 1360


Alleles, Animals, Female, Genetic Linkage, Mice, Oxidoreductases, N-Demethylating, Proteins, X Chromosome