One class of mutations affects the gene CDKN2A. An alternative reading frame mutation in this gene leads to the destabilization of p53, a transcription factor involved in apoptosis and in fifty percent of human cancers. Another mutation in the same gene results in a nonfunctional inhibitor of CDK4, a cyclin-dependent kinase that promotes cell division. Mutations that cause the skin condition xeroderma pigmentosum (XP) also increase melanoma susceptibility. Scattered throughout the genome, these mutations reduce a cell's ability to repair DNA. Both CDKN2A and XP mutations are highly penetrant (the chances of a carrier to express the phenotype is high).