Alkaptonuria is a rare familial disease of organic acid metabolism that is best known for the darkening of urine from yellow to brown to black after it is exposed to the air. In later life, individuals with this disease develop arthritis characterized by deposition of brown pigment in joint cartilage and connective tissue. Garrod studied the recurrence patterns in several families, realized it followed an autosomal recessive pattern of inheritance, and postulated that it was caused by a mutation in a gene encoding an enzyme involved in the metabolism of a class of compounds called alkapton. He published "The Incidence of Alkaptonuria: a Study in Chemical Individuality" in 1902.