RB1 and neoplasm: In various human malignant cell lines, in which it is likely that the p16-pRB pathway has been compromised by either genotoxic or PcG-mediated silencing, and in which MLL1 complex activity is increased, gene knockdown experiments have shown that MLL1 is a key player in maintaining several aspects of the malignant phenotype, including cell proliferation, tumour growth, hypoxia signaling and angiogenesis in vivo.