Cells undergoing apoptosis lose the asymmetry of plasma membrane phospholipids, and phosphatidylserine is exposed on the outer surface of the membrane. The phosphatidylserine receptor (PSR) specifically recognizes phosphatidylserine and this binding triggers the phagocytosis of apoptotic cells by either macrophages or dendritic cells. PSR is expressed on the surface of macrophages, fibroblasts, and epithelial cells, and it has been detected in high levels in heart, skeletal muscle, and kidney tissues and is extensively glycosylated. The mammalian phosphatidylserine receptor displays significant homology to Caenorhabditis elegans and Drosophila melanogaster proteins, which suggests that PSR has been conserved throughout phylogeny.