Papular epidermal nevus with "skyline" basal cell layer (PENS) is a rare type of epidermal nevus with the distinctive histopathologic finding of a palisaded arrangement of basal cell nuclei, resembling a skyline. PENS is characterized clinically by single or multiple small, polygonal or comma-shaped hyperkeratotic papules that appear at birth or shortly thereafter. Lesions may occur in a randomized pattern or with a Blaschko-linear distribution. PENS lesions tend to be stable throughout life and are asymptomatic.

While most cases of PENS develop sporadically, familial occurrence has been described and a genetic basis suspected.

PENS syndrome (PENSS) refers to a rare neurocutaneous syndrome characterized by the presence of typical PENS lesions in association with extracutaneous findings, mainly neurologic. PENSS has been reported to occur in up to 50% of PENS cases. It has been postulated that individuals with more than 4 PENS lesions are at greater risk of also having extracutaneous manifestations. Neurological signs are of varying severity and may include psychomotor delay, learning difficulties, dyslexia, hyperactivity, attention deficit disorder, autism spectrum disorder, and epilepsy. Neurologic manifestations may occur within the first 1-2 years of age. Some neurologic manifestations have been noted to improve and/or resolve during infancy and adolescence. Other extracutaneous manifestations such as dysmorphic facies, Achilles tendon shortening, hypospadias, and curved penis have also been described.