As for most chromosome X genes in rodent extra-embryonic tissues, Esx1 (Extraembryonic,
spermatogenesis, homeobox 1), is maternally expressed, as a result of imprinted silencing of the
paternal chromosome X. Mutant mice have large placentas but are 20% smaller because of placental
Y and Behringer RR, 1998).
Traditionally the 1000 or so chromosome X genes that exhibit maternal expression in extra-embryonic
tissues are not counted among the imprinted genes (with the exception of Xist and Tsix).