A marsupial (Sminthopsis douglasi) with bilateral intersexuality had a hemiscrotum on the right side and a hemi-pouch with nipples on the left. A normal female karyotype (2n = 14, XX) was present in cells from the right (male) side, while cells from the left (female) side initially had a female karyotype plus two dot-like chromosomes (2n = 14, XX + 2B). It is proposed that the dots represented a region deleted from the X chromosome that contains the "pouch-mammary/scrotum" (PMS) switch gene whose dosage determines development of a pouch and teats (two doses) or a scrotum (one dose). Mis-segregation early in embryonic development produced a lineage with one normal X and one deleted X (male side), and a lineage with a normal and deleted X, plus two copies of the deleted region (female side). The origin of the supernumerary elements was therefore investigated in the expectation that they may contain the long-sought pouch-mammary/scrotum switch gene. Several elements were microdissected, and amplified DNA was used for in situ hybridization, producing signals in five different chromosome regions including the X. This could represent a region of the X that contains, as well as PMS, repetitive DNA that is present also at other chromosomal sites.