Somatic cell nuclear transfer has many potential applications in the fields of basic and applied sciences. However, it has a disadvantage that can never be overcome technically-the inflexibility of the sex of the offspring. Here, we report an accidental birth of a female mouse following nuclear transfer using an immature Sertoli cell. We produced a batch of 27 clones in a nuclear transfer experiment using Sertoli cells collected from neonatal male mice. Among them, one pup was female. This "male-derived female" clone grew into a normal adult and produced offspring by natural mating with a littermate. Chromosomal analysis revealed that the female clone had a 39,X karyotype, indicating that the Y chromosome had been deleted in the donor cell or at some early step during nuclear transfer. This finding suggests the possibility of resuming sexual reproduction after a single male is cloned, which should be especially useful for reviving extinct or endangered species.