Phospholipid scramblase that promotes phosphatidylserine exposure on apoptotic cell surface (PubMed:25231987). Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment (PubMed:25231987). 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 9, processed form. Does not act prior the onset of apoptosis. Undergoes proteolytic processing by caspase-3 (CASP3), caspase-6 (CASP6) and caspase-7 (CASP7) to generate the XK-related protein 9, processed form, leading to its activation. Belongs to the XK family.