Mitochondrial isovaleryl dehydrogenase (IVD) catalyzes the reaction of isovaleryl-CoA and FAD to form beta-methylcrotonyl-CoA and FADH2 (Finocchiaro et al. 1978; Rhead and Tanaka 1980). Crystallographic studies demonstrated the existene of a tetramer of IVD polypeptides lacking an aminoterminal mitochondrial targeting sequence (Tiffany et al. 1997).