CDKN1B and teratoma: Although the involvement of p27kip1 and p21cip1, which encode the CDK inhibitors p27kip1 and p21cip1, in the regulation of teratoma formation is currently unclear, in embryonic testes, the expression of p27kip1 and p21cip1 causes embryonic germ cells to enter a period of mitotic arrest at E14.5 (Western et al., 2008).