Phospholipid scramblase that promotes phosphatidylserine exposure on apoptotic cell surface. Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment. 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 (By similarity). Promotes myoblast differentiation and survival (By similarity). 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. Does not act prior the onset of apoptosis. Interacts with BSG and NPTN; which act as chaperones to localize XKR8 at the cell membrane. Homodimer. Undergoes proteolytic processing by caspase-3 (CASP3), leading to its activation. Phosphorylation at Thr-375 activates the phospholipid scramblase activity. Belongs to the XK family.