Our results showed differences between the “transitioned” tumors (AD to SCLC) and the combined tumors (CSCLC): the former often had the same EGFR mutations before and after the transition, while the latter expressed RB1 and EGFR in the NSCLC component more frequently than that in the SCLC component, EGFR and KRAS mutation was found in 1 but not both components of CSCLC. The gene discussed is KRAS; the disease is small cell lung carcinoma.