RB1 and primitive neuroectodermal tumor: For example, Chiu and colleagues demonstrated that mice possessing an SV40 T-antigen transgene (which functionally inactivates Rb and p53), driven by the brain specific FGF-1B promoter, develop poorly differentiated tumors of the medulla and 4th ventricle which closely resemble primitive neuroectodermal tumors (PNET) [42].