The best known FA undergoing alpha-oxidation, is phytanic acid (3,7,11,14-tetramethylhexadecanoic acid) as concluded from observations on a rare disease called Refsum disease in which alpha-oxidation is blocked due to a genetic deficiency of the enzyme phytanoyl-CoA 2-hydroxylase encoded by PHYH. Phytanic acid is strictly derived from dietary sources and cannot be synthesized de novo. This evidence concerns the gene PHYH and Refsum disease.