Overview of DHA biosynthesis in liver. Diet-derived ALA (C18:3 omega-3) is transformed into tetracosahexaenoic acid (C24:6 omega-3) by the sequential action of Δ6 and Δ5 desaturases (encoded by the FADS2 and FADS1 genes, respectively) and elongases (such as that encoded by the HELO1 gene) present in the endoplasmatic reticulum. Tetracosahexaenoic acid is transported into peroxisomes (shaded area), presumably by proteins encoded by the ABCD1 or ABCD2 genes, and then converted into docosahexaenoic acid (DHA, C22:6 omega-3) by sequential action of acyl coenzyme-A oxidase (encoded by the ACOX1 gene), d-bifunctional protein (encoded by the HSD17B4 gene), and various peroxisomal thiolases (not shown). The figure shows chemical structures of fatty acids quantified in our analyses.