Phospholipid scramblase that promotes phosphatidylserine exposure on apoptotic cell surface (PubMed:23845944, PubMed:25231987). Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment (PubMed:23845944). 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 (By similarity). Has no effect on calcium-induced exposure of phosphatidylserine (PubMed:23845944). Promotes myoblast differentiation and survival (PubMed:28881496). (Microbial infection) Incorporated into Ebola virus-like particles, where its phospholipid scramblase activity is required to promote phosphatidylserine exposure on the surface of viral particles (PubMed:29338048). Externalization of phosphatidylserine on the surface of viral particles is required for uptake by host cells (PubMed:29338048). 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 (PubMed:23845944). Does not act prior the onset of apoptosis (PubMed:23845944). Interacts with BSG and NPTN; which act as chaperones to localize XKR8 at the cell membrane. Homodimer. Undergoes proteolytic processing by caspase-3 (CASP3) to generate the XK-related protein 8, processed form, leading to its activation. Phosphorylation at Thr-375 activates the phospholipid scramblase activity. Expression is repressed in PLB-985 leukemia and Raji lymphoma cells due to CpG methylation near the transcription start site of XKR8 gene, possibly explaining the inability of PLB-985 leukemia and Raji lymphoma cells to expose phosphatidylserine during apoptosis. Belongs to the XK family.