p14ARF (CDKN2A, cyclin-dependent kinase inhibitor 2A, ARF isoform) is an alternative-reading-frame product of tumor-suppressor CDKN2A, responsible for p53 stabilization, cell-cycle arrest and oncogenic surveillance. Translated from distinct reading frame of CDKN2A, p14ARF does not directly inhibit cyclin-dependent kinases. It binds and sequesters MDM2 to block MDM2-dependent p53 ubiquitination-degradation, elevating p53 abundance to trigger cell-cycle arrest and apoptosis upon oncogenic or genotoxic stress. Oncogenic activation up-regulates p14ARF as a critical tumor-suppressive checkpoint. Frequent locus deletion and epigenetic silencing of CDKN2A occur in diverse human malignancies.