A degenerative disease of the brain that causes gradual loss of memory, judgment, and the ability to function socially. About 25% of all Alzheimer disease is familial (more than 2 people in a family have AD). When Alzheimer disease begins before 60 or 65 years of age (early-onset AD) about 60% of the cases are familial (also known as Early-onset familial AD). These cases appear to be inherited in an autosomal dominant manner.