Although neither Ntv-a Ink4a/Arf+/-Pten+/fl nor Ntv-a Ink4a/Arf-/-Ptenfl/fl mice generate spontaneous gliomas, it is likely that experimentally provided genetic hits may represent the rate limiting step for transformation and thus, transformation of recruited cells may be less common in human gliomas. The gene discussed is PTEN; the disease is central nervous system cancer.