Phospholipid scramblase that promotes phosphatidylserine exposure on apoptotic cell surface (PubMed:23845944, PubMed:25231987, PubMed:27503893, PubMed:29440417, PubMed:30718401). Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment (PubMed:23845944, PubMed:29440417, PubMed:30718401, PubMed:31712393). Required for the clearance of apoptotic cells, such as engulfment of apoptotic germ cells by Sertoli cells, clearance of senescent neutrophils or regulation of bipolar cell numbers in the retina (PubMed:30559640, PubMed:29440417, PubMed:31712393). Has no effect on calcium-induced exposure of phosphatidylserine (By similarity). Promotes myoblast differentiation and survival (PubMed:28881496). a 1,2-diacyl-sn-glycero-3-phospho-L-serine(in) = a 1,2-diacyl-sn-glycero-3-phospho-L-serine(out) Activated upon caspase cleavage to generate the XK-related protein 8, processed form (By similarity). Does not act prior the onset of apoptosis (By similarity). Interacts with BSG and NPTN; which act as chaperones to localize XKR8 at the cell membrane. Homodimer. Widely expressed with higher expression in testis (PubMed:23845944). Expressed in the bone marrow and mononuclear leukocytes in the blood; highly expressed in thymocytes and lymphocytes (PubMed:29440417). Expressed in the mature retina (PubMed:30559640). Undergoes proteolytic processing by caspase-3 (CASP3), leading to its activation. Phosphorylation at Thr-375 activates the phospholipid scramblase activity. Conditional knockout mice show defects in phosphatidylserine exposure following apoptosis, leading to defects in engulfment of apoptotic cells (PubMed:23845944, PubMed:29440417). Mice on the MRL background develop a lupus-like autoimmune disease caused by impaired clearance of apoptotic lymphocytes and aged neutrophils (PubMed:29440417). Male mice are infertile due to reduced sperm counts in their epididymides: defects are caused by inefficient clearance of apoptotic germ cells by Sertoli cells in testes (PubMed:31712393). Belongs to the XK family.