S-Adenosylhomocysteine (SAH) Hydrolase Deficiency (Hypermethioninemia, familial) is caused by a defect in the AHCY gene, which codes for S-adenosylhomocysteine hydrolase (SAH). S-adenosylhomocysteine hydrolase catalyzes the hydrolysis of S-adenosylhomocysteine to adenosine and homocysteine. In eukaryotes, this is the major route for disposal of the S-adenosylhomocysteine formed as a common product of each of many S-adenosylmethionine-dependent methyltransferases. SAH Deficiency causes accumulation of guanidinoacetate, homocysteine, methionine, s-adenosylhomocysteine and s-adenosylmethionine in plasma, and methionine in spinal fluid. Symptoms include cerebral atrophy, dysmorphism, strabismus, jaundice, mental and motor retardation.