What is HCU :

People with Homocystinuria (HCU)  cannot digest the amino acid called Methionine. HCU,  is similar to  Phenylketonuria (PKU),  Methylmalonic Acidemia(MMA)  and Tyrosinemia. Each of these disorders require food products which are low in particular types of amino-acids. i.e. Methonine in the case of HCU, Phenylalanine in the case of PKU.

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Detailed Explanation:

Homocystinuria is an inborn error of methonine metabolism and was discovered in 1962 independently in Northern Ireland by Carson and Neill and in the United States by Gerritsen et al. The basic defect was defined by Mudd and his co-workers in 1964 as a deficiency of the hepatic enzyme Cystathionine ß Synthase. ( Nina A.J. Carson, The Treatment of Inherited Metabolic Disorders)