BMP15 and osteochondritis dissecans: Alternatively, given that genes in the vicinity BMP15 are subject to X-inactivation (35) and silencing of one X often occurs in a block-wise and skewed manner (36), it is possible that the copy of BMP15 containing the frameshift variant became preferentially silenced in Orcadian females, whereas the missense mutation was expressed to a greater extent in the heterozygous females who developed OD.