Neurocutaneous melanosis.

Neurocutaneous melanosis (NCM) is a rare phakomatosis characterized by a focal or diffuse proliferation of melanin-producing cells in both the skin and the leptomeninges. This syndrome is believed to result from an error in the morphogenesis of embryonal neuroectoderm. Two-thirds of patients with NCM have giant congenital melanocytic nevi, and the remaining one-third have numerous lesions but no giant lesions.