Studies on mice lacking PPAR alpha, peroxisomal fatty acyl-CoA oxidase, and certain enzymes in the two peroxisomal beta-oxidation pathways highlight the crucial roles of PPAR alpha and classical peroxisomal fatty acyl-CoA oxidase in energy metabolism, as well as in the inception of some diseases such as hepatic steatosis, or liver cancer [24]. This evidence concerns the gene ACOX1 and fatty liver disease.