p53 acts as both a tumor-suppressor and transcription factor that, upon activation by DNA damage and other cellular stress signals, leads to the transcription of genes triggering cell-cycle arrest, apoptosis, and DNA repair. p53 is overexpressed in over 50% of human cancers. Positive staining of p53 detected by immunohistochemistry has been observed in colon cancer, breast cancer, lung cancer, prostate cancer and ovarian cancer.