Phosphatidylserine (PS) accounts for 5 to 10% of cell membrane phospholipids. In addition to its role as a structural component, PS is involved in cell signaling, blood coagulation, and apoptosis. PS is synthesized by a calcium-dependent base-exchange reaction catalyzed by PS synthases (EC 2.7.8.8), like PTDSS2, that exchange L-serine for the polar head group of phosphatidylcholine (PC) or phosphatidylethanolamine (PE) (Sturbois-Balcerzak et al., 2001 [PubMed 11084049]).