p53 is a nuclear phosphoprotein with a molecular mass of 53 kDa. Wild-type p53 protein is present in a wide variety of normal cells, but the protein has a very short half-life and thus is present in only minute amounts, generally below the detection level of immunohistochemical methods. Somatic mutation of the p53 gene is very frequent event in the development of human neoplasia, and because mutant p53 proteins often are much more stable than wild-type protein, the mutant p53 protein accumulates to a high level. As examples, p53 protein accumulations were observed in 76% of 212 human malignant lesions, including breast, colon and stomach carcinomas, melanoma, embryonal carcinoma of the testis, transitional carcinoma of the urinary bladder, uterine carcinoma and soft tissue sarcomas. Wild -type p53 protein functions as a transcription factor, i.e. as a modulator which can turn crucial genes either on or off. It also inhibits DNA replication and is a check-point control molecule for progression of the cell cycle. Furthermore, p53 protein is involved in the regulation of apoptosis. In transfection assays, wild-type p53 behaves as a tumor suppressor, while mutant p53 behaves as a dominant transforming oncogene.