Phosphatidylserine (PS) is mainly present in the cytosolic leaflet of plasma membrane, endosomes and lysosomes. It is also present in the interior of the endoplasmic reticulum, Golgi complex and mitochondria. PS is the preferential binding substrate for Clostridium sordellii lethal toxin (TcsL). PS is a component of the lipid-calcium-phosphate complex for deposition during bone formation, regulation of apoptosis and blood coagulation.