Exceptions to these protein families are found for salmonids, where sdy is the MSD gene [9] and recently the breast cancer anti-estrogen resistance protein 1 (BCAR1) and bone morphogenetic protein receptor type-1B (bmpr1b) have been reported as MSD genes in catfish (Ictalurus punctatus) [10] and Atlantic herring (Clupea harengus) [11], respectively. The gene discussed is BMPR1B; the disease is breast cancer.