Orphanet_6 (Isolated 3-methylcrotonyl-CoA carboxylase deficiency) can be described as follows. 3-methylcrotonyl-CoA carboxylase deficiency (3-MCCD) is an inherited disorder of leucine metabolism characterized by a highly variable clinical picture ranging from metabolic crisis in infancy to asymptomatic adults. Also known as: 3-methylcrotonylglycinuria, MCC deficiency, MCCD.