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. This evidence concerns the gene CDKN2A and central nervous system cancer.