The INK4a-ARF locus is comprised of two tumor suppressors, p16INK4a and p14ARF. These two proteins are encoded through differential splicing of alternative first exons. The p16INK4a (exon 1alpha) protein inhibits the cyclin D-dependent kinases (CDK) that control the phosphorylation of the Rb protein and cell proliferation. The p14ARF gene product complexes with the MDM2 protein within the nucleus, thus modulating the activity of the p53 protein. P14ARF is a potent tumor suppressor in the presence of wild-type p53, while mutant p53 substantially reduces growth inhibition by p14ARF.