Clear cell acanthoma developing in epidermal nevus

A 33-year-old Japanese woman presented with a black papule on a pigmented lesion which had been on her right thigh since her early childhood. A hematoxylin-eosin-stained section revealed a sharply demarcated, acanthotic epidermis composed of enlarged clear cells, which stained positively for epithelial membrane antigen and negatively for carcinoembryonic antigen. With antikeratin antibodies, the tumor cells stained for AE1 and AE3, but did not stain for CAM5.2. They contained abundant glycogen. Histologically, we diagnosed the case as a clear cell acanthoma which developed in the pre-existing epidermal nevus. This is the second such case in the literature.