3-Hydroxyacyl-CoA dehydrogenase (HAD) functions in mitochondrial fatty acid b-oxidation by catalyzing the oxidation of straight chain 3-hyd-roxyacyl-CoAs. HAD has a preference for medium chain substrates, whereas short chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) acts on a wide spectrum of substrates, including steroids, cholic acids, and fatty acids, with a preference for short chain methyl-branched acyl-CoAs.