Inactivation of the RB1 and TP53 tumor suppressor genes is observed in nearly all human SCLC cases (3, 5, 6), which led to the development of the Rb1fl/fl; Trp53fl/fl (RP) genetically engineered mouse model (GEMM) for SCLC. The gene discussed is RB1; the disease is small cell lung carcinoma.