MRC1 and keratoacanthoma: In lesions for which IHC (n = 253) was undertaken, 239 (94%) demonstrated MMR protein loss corresponding to their underlying germline genetic defects, but a minority of skin lesions (n = 14, 5%) were found to be MMR proficient, including 5 Sebaceous Adenomas, 2 keratoacanthoma, 2 SCC, 1 Melanoma and 4 others.