Genetic encoding and mechanism of action of tumor suppressors p16^INK4a and p14^ARF. The CDKN2A locus on chromosome 9p21 is composed of 4 exons (E) – 1α, 1β, 2, and 3 – and encodes 2 tumor suppressors, p16^INK4a and p14^ARF (termed p19^ARF in the mouse), via alternative reading frames. p16^INK4a is translated from the splice product of E1α, E2, and E3, while p14^ARF is translated from the splice product of E1β, E2, and E3. Normally p16^INK4a sequesters CDK4 and CDK6 thereby keeping pRb in an active hypophosphorylated state. In the absence of p16^INK4a, CDK4 and CDK6 bind cyclin D and phosphorylate Rb. Phosphorylated pRb then releases the E2F transcription factor, which promotes the G1-to-S phase transition of the cell cycle. p14^ARF, on the other hand, sequesters the p53-specific ubiquitin ligase HDM2. In the absence of p14^ARF, HDM2 targets p53 for ubiquitination (UUU) and subsequent proteosomal degradation, and the loss of p53 impairs mechanisms that normally target genetically damaged cells for cell cycle arrest and/or apoptosis, which leads to proliferation of damaged cells. Loss of CDKN2A therefore contributes to tumorigenesis by disruption of both the pRb and p53 pathways. Figure modified with permission from N. Engl. J. Med. (S30).