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Nevus sebaceous is an extremely common hamartoma composed of mixed epithelial, sebaceous, and apocrine gland components. It is a component, with accompanying skeletal, cerebral, or ocular defects, of Schimmelpenning syndrome.
Groesser and colleagues studied a series of 65 nevi sebaceous lesions; they found that 91% harbor the HRAS mutation c.37G>C. In one case, a syringocystadenoma papilliferum had the same mutation as the nevus sebaceous from which it arose. Neighboring keratinocytes and germline sequences were normal. The mutation occurred at a highly conserved site; it locks RAS in an active state, promoting proliferation and survival. Another mutation, c.34G>T, which causes similar biochemical effects, is the same mutation found in the g…