Mitochondrial fatty acid oxidation deficiencies are due to genetic defects in enzymes of fatty acidb-oxidation and transport defects have been identified in most of the genes where nearly all types of sequence vari-ations (mutation types) have this paper, we will discuss the effects of the various types of sequence variations encountered and review current know-ledge regarding the genotype–phenotype relationship, espe-cially in patients with acyl-CoA dehydrogenase deficiencies where sufficient material exists for a meaningful discussion