Parthenogenesis has been suggested to be tightly coupled with development of ovarian teratomas. Indeed, ovarian tumors developed in c-mos-deficient female mice, which are characterized by the parthenogenetic activation of oocytes. The tumors appeared at a frequency of 30% between 4 and 8 months of age, and did not develop in younger or older mice. Most of the tumors were benign and consisted of multi-focal cysts most notably with mature ectodermal components, but also with mesodermal and endodermal components. One among 17 tumors observed consisted of extra-embryonic tissues alone, and two bore malignant components with metastasis to peritoneal organs. The results strongly suggest the involvement of c-mos mutations in human germ cell tumors.