Mullerian Duct Syndrome (MDS) is an inherited disorder of sexual development affecting male dogs. In early in-utero development all canine fetuses have precursors of the uterus, fallopian tubes and upper vagina called Mullerian ducts. In normal male fetuses, the Mullerian ducts regress as sexual differentiation occurs in-utero, allowing for development of male sexual anatomy. In affected male dogs the female reproductive organs fail to regress in-utero. Approximately half of the affected male dogs have externally normal testes and are fertile, but internally have remnants of female reproductive organs, including the uterus. The other 50% of affected male dogs have Cryptorchidism in addition to remnants of female reproductive structures. Cryptorchidism predisposes dogs to infertility and testicular tumors. Affected dogs may also have small testes. Female dogs may need to have a Hysterectomy if the uterus becomes infected. Females with this condition have normal internal and external female anatomy.