DHA levels are not homogeneously distributed throughout an animal. There are two distinct categories, select tissues containing large amounts of DHA and the remaining tissues with normally less than a few mol%. High-DHA tissues, the rod outer segment, sperm, and brain have a large quantity of DHA. DHA is mainly found on in the sn-2 position of phospholipids. DHA is n-3 fatty acid that is essential for normal brain development and cognitive function. It was confirmed a member of the major facilitator superfamily: Mfsd2a as the major transporter for DHA uptake into brain. Mfsd2a is found to be expressed exclusively in endothelium of the blood-brain barrier of micro-vessels. Mfsd2a-KO mice show markedly reduced levels of DHA in brain accompanied by neuronal cell loss in hippocampus and cerebellum. Mfsd2a transports DHA in the form of sn-2 lysophosphatidylcholine, but not unesterified fatty acid, in a sodium ion-system. A remarkable property of MFSD2 is its high-level expression specifically in the placenta, with expression in all other tissues at least 10-fold low―with the expression of the testis in which it is only 4-fold lower than the placenta. DHA is specifically bound to fatty acid binding proteins (Fabp7) in the cytoplasm, and transported to the cell membrane and work as ligands for nuclear receptor transcription factor.