Phospholipid scramblase that promotes phosphatidylserine exposure on apoptotic cell surface (PubMed:25231987, PubMed:33725486). Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment (PubMed:25231987, PubMed:33725486). a 1,2-diacyl-sn-glycero-3-phospho-L-serine(in) = a 1,2-diacyl-sn-glycero-3-phospho-L-serine(out) Phospholipid scramblase activity is activated upon caspase cleavage to generate the XK-related protein 4, processed form (PubMed:25231987, PubMed:33725486). Does not act prior the onset of apoptosis (PubMed:25231987). Homodimerizes upon caspase cleavage (PubMed:33725486). Phospholipid scramblase activity is activated following interaction with the processed C-terminus of XRCC4 (protein XRCC4, C-terminus) (PubMed:33725486). Homodimer; homodimerization takes place upon caspase cleavage (PubMed:33725486). Interacts with the processed C-terminus of XRCC4 (protein XRCC4, C-terminus); interaction promotes the phospholipid scramblase activity (PubMed:33725486). Highly expressed in expressed in the brain; weakly expressed in the spleen, thymus, uterus, blood vessels and fetus. Undergoes proteolytic processing by caspase-3 (CASP3), caspase-6 (CASP6) and caspase-7 (CASP7) to generate the XK-related protein 4, processed form, leading to its activation. Belongs to the XK family.