CDKN2A and cancer: Indeed, although RB1 mutations are extremely rare in this cancer type (COSMIC, the Catalogue of Somatic Mutations in Cancer, http://cancer.sanger.ac.uk), the homozygous deletion of the cyclin-dependent kinase inhibitor 2A (CDKN2A) locus, which results in RB1 functional inactivation, is one of the most common mutations in MM cells [19,20], including the NCI-H28 and MSTO-211H cell lines under study (COSMIC).