PTEN and teratoma: Support for this hypothesis is provided by the strong evidence that spontaneous differentiation of ESCs in vitro consistently generates a small population of germ cells within 3–5 days of culture [27], [28], [29], [30], [31], [32], together with the result that conditional loss of Pten in fetal germ cells invariably causes teratomas from the abnormal germ cells in vivo[33].