Xk-related protein 4 (Xkr4) protein is one of the scramblases that can expose phosphatidylserine (PS) as an “eat-me” signal in apoptotic cells. Xkr4 is activated in two steps: Dimer formation after caspase-mediated cleavage of its cytoplasmic region, followed by direct binding of the C-terminal fragment of X-Ray Repair Cross Complementing 4 (XRCC4/C-terminus) to the Xkr4 dimer. It has b…