Mutations in TP53 are, therefore, more advantageous and more regularly achieved than PTCH2 LOH which, in turn, could explain the low mutation rate of PTCH2. Another explanation for the paucity of PTCH2 mutations in human BCC follows directly from our observation that full Ptch2 gene ablation could not be achieved, despite selection. This evidence concerns the gene TP53 and skin basal cell carcinoma.