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. Isoform 1 has no prospholipid scramblase activity, due to the lack of a N-terminal proline-rich domain. Expression of isoform 1 seems restricted to testis. The N-terminal proline-rich domain (PRD) is required for phospholipid scramblase activity. Belongs to the phospholipid scramblase family. Extended N-terminus. Scramblase entry