May mediate accelerated ATP-independent bidirectional transbilayer migration of phospholipids upon binding calcium ions that results in a loss of phospholipid asymmetry in the plasma membrane. May play a central role in the initiation of fibrin clot formation, in the activation of mast cells and in the recognition of apoptotic and injured cells by the reticuloendothelial system. Expressed in erythrocyte membranes, blood platelets, blood leukocytes, T- and B-lymphocytes, vascular endothelium, spleen, thymus, prostate, testis, uterus, intestine and colon. The N-terminal proline-rich domain (PRD) is required for phospholipid scramblase activity. Belongs to the phospholipid scramblase family.