Tumor protein p63 is a member of the p53 family of transcription factors. The functional domains of p53 family proteins include an N-terminal transactivation domain, a central DNA-binding domain and an oligomerization domain. Alternative splicing of this gene and the use of alternative promoters results in multiple transcript variants encoding different isoforms that vary in their functional properties. These isoforms function during skin development and maintenance, adult stem/progenitor cell regulation, heart development and premature aging. P63 is an extremely sensitive marker of lung squamous cell carcinoma, but it suffers from low specificity due to its reactivity in a substantial proportion of lung adenocarcinomas and other tumor types. P40 is a splice isoform of p63 known as DeltaNp63 and has been demonstrated to be highly specific for squamous/basal cells making it is markedly superior to p63 in specificity [adapted from NCBI Entrez Gene (Gene ID: 8626)].