RB1 and small cell lung carcinoma: The authors also evaluated the co-occurrence and mutual exclusivity of other genetic alterations in SCLC tumors with wild-type TP53 and/or RB1 in comparison with TP53- and RB1-mutant SCLC tumors, and found that loss-of-function mutations in CDKN2A, encoding the positive modulator of Rb, namely p16, as well as gain-of-function mutations CCND1, encoding the negative modulator of Rb, namely cyclin D1, showed mutual exclusivity with genomic aberrations in RB1.