Combined methylmalonic aciduria (MMA) and homocystinuria is a genetically heterogeneous metabolic disorder of cobalamin (cbl; vitamin B12) metabolism, which is essential for hematologic and neurologic function. Biochemically, the defect causes decreased levels of the coenzymes adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl), which results in decreased activity of the respective enzymes methylmalonyl-CoA mutase and methyltetrahydrofolate:homocysteine methyltransferase, also known as methionine synthase. The cblJ type is phenotypically and biochemically similar to the cblF type. [From OMIM]