RB1 and neoplasm: CDKN2A is localized in chromosome 9p21.3 and encodes two tumor-suppressor proteins that regulate the activities of p53 and pRB (encoded by TP53 and RB1 genes, respectively) in tumor suppression; ARF (alternate reading frame; aka p14ARF in human) triggers p53-mediated cell-cycle arrest or apoptosis by inactivating MDM2 (mouse double minute 2), an E3 ubiquitin-protein ligase targeting p53 for destabilization, whereas INK4a (aka p16INK4a) promotes pRB-mediated cell-cycle checkpoints by inhibiting CDK4 (cyclin d-dependent kinase 4) that phosphorylates and inactivates pRB [9].