TP53 and squamous cell carcinoma: Precursor lesions such as actinic keratosis (AK) is associated with p53 mutations (and moderately increased p16 expression); squamous cell carcinoma (SCC) is associated with p53 mutations, increased p16 expression, activation of the mitogenic ras pathway, reduced expression of FasR (CD95-R) and increased expression of FasL; and basal cell carcinoma (BCC) is associated with mutations in PTCH (from the sonic hedgehog pathway) and p53. The contiguous nature and ease of accessibility of skin has made this organ suitable for studies of the mechanisms of how field cancerization develops.