MCCC1 gene encodes the alpha subunit of a mitochondrial enzyme called the 3-methylcrotonoyl-CoA carboxylase (3-MCC), which plays an important role in leucine and isovaleric acid catabolism. Defects in this enzyme caused by mutations in MCCC1 gene lead to build-up of uncatalyzed compounds of leucine metabolic pathway in the body, resulting in the characteristic features of 3-MCC deficiency.