ATP7A and Menkes disease: About 370 mutations have been identified in the ATP7A gene in humans, resulting in 3 different clinical phenotypes of Menkes disease: (1) Classical Menkes disease: patients with severe symptoms including mental retardation who die in early childhood (before the age of 6 years); (2) atypical Menkes disease: patients with less severe symptoms who live until adolescence; and (3) Occipital Horn Syndrome: patients with normal or near-normal neurological functions, milder symptoms, predominantly connective tissue problems, who live up to mid-adulthood.