One possibility is that, like some other X-linked genes, silencing of the Atrx gene on Xp is incomplete, such that there is always a low-level, leaky output of Atrx from a normally inactivated Xp chromosome in extraembryonic tissues. However, it was recently demonstrated that the paternal Atrx (called Xnp) allele is completely silenced in a normal mouse trophoblast stem cell line [32], suggesting that Atrx does not normally escape imprinted X-inactivation in the extraembryonic tissues of wild-type females. Thus, the expression of the Xp-linked Atrx WT allele that we observed is unique to female carriers of the Atrx null allele.