SCP2 and Refsum disease: In Refsum disease and RCDP type 1 and 5, the major accumulating metabolite is phytanic acid due to a defect of alpha-oxidation, whereas in DBP, AMACR and SCPx deficiency the pristanic/phytanic acid ratio is increased because peroxisomal beta-oxidation of pristanic acid is impaired and phytanic acid accumulation is only secondary.