What to know about homocystinuria Homocystinuria, an inherited disorder, is a rare metabolic condition that impacts 1 in 335,000 people worldwide and is present from birth. Often abbreviated as HCU or HCY, homocystinuria is a genetic disorder that inhibits the body from processing certain amino acids found in proteins, causing a harmful buildup of homocysteine and methionine. As a result of this harmful buildup, children with homocystinuria often have a fair complexion, struggle with weight...