Basal cell carcinoma: pathophysiology. 
Basal cell carcinoma (BCC) is the most common skin cancer in humans, which typically appears over the sun-exposed skin as a slow-growing, locally invasive lesion that rarely metastasizes.  Although the exact etiology of BCC is unknown, there exists a well-established relationship between BCC and the pilo-sebaceous unit, and it is currently thought to originate from pluri-potential cells in the basal layer of the epidermis or the follicle.  The patched/hedgehog intracellular signaling pathway plays a central role in both sporadic BCCs and nevoid BCC syndrome (Gorlin syndrome).  This pathway is vital for the regulation of cell growth, and differentiation and loss of inhibition of this pathway is associated with development of BCC.  The sonic hedgehog protein is the most relevant to BCC; nevertheless, the Patched (PTCH) protein is the ligand-binding component of the hedgehog receptor complex in the cell membrane.  The other protein member of the receptor complex, smoothened (SMO), is responsible for transducing hedgehog signaling to downstream genes, leading to abnormal cell proliferation.  The importance of this pathway is highlighted by the successful use in advanced forms of BCC of vismodegib, a Food and Drug Administration-approved drug, that selectively inhibits SMO.  The UV-specific nucleotide changes in the tumor suppressor genes, TP53 and PTCH, have also been implicated in the development of BCC.