Acne arising in an epidermal nevus.

Epidermal nevi are benign congenital lesions that follow Blaschko lines. Epidermal nevi can demonstrate a variety of histologic patterns and contain hamartomatous hyperplasia of any epidermal or adnexal structure. We present the first reported instance of acne arising strictly within a preexisting keratinocytic epidermal nevus during puberty, which is a demonstration of genetic mosaicism in pilosebaceous units.