is the bluish-black discoloration of certain tissues, such as the ear cartilage, the ocular (eye) tissue, and other body locations. It often occurs in individuals with alkaptonuria, a metabolic disorder; it can also occasionally occur from exposure to various substances. Alkaptonuria is caused by a mutation in the HGD gene, which results in the accumulation and deposition of homogentisic acid (HGA) in cartilage, causing the characteristic coloring of ochronosis. The affected tissue becomes weak and brittle with time, leading to chronic inflammation, degeneration, and osteoarthritis. Alkaptonuria is inherited in an autosomal recessive manner. When ochronosis is due to exposure to substances (called exogenous ochronosis), the condition is not inherited. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.