Phosphatidylserine is a naturally occurring phospholipid that comprises 2-10% of total phospholipids in mammals and is enriched in the central nervous system, particularly the retina. It is anionic and found mainly on the inner leaflet of the cell membrane. It is biosynthesized from phosphatidylcholine or phosphatidylethanolamine by phosphatidyl synthase 1 (PSS1) or PSS2, respectively, in the endoplasmic reticulum and can be reversibly converted back by the same enzymes. It can also be irreversibly converted to phosphatidylethanolamine by phosphatidylserine decarboxylase in the mitochondria. Phosphatidylserine binds to T cell immunoglobulin mucin type 1 (TIM-1) and TIM-4 receptors as well as brain-specific angiogenesis inhibitor 1 (BAI1), leading to anti-inflammatory and anti-atherosclerotic effects. It is also a cofactor involved in the activation of various signaling pathways through activation of protein kinase C, neutral sphingomyelinase, and c-Raf-1 protein kinase among others. Phosphatidylserine is externalized during apoptosis by scramblases in the plasma membrane as a signal for phagocytes to engulf the cell. Phosphatidylserines (bovine) is a mixture of bovine phosphatidylserines containing fatty acids with variable chain lengths at the sn-1 and sn-2 positions.