Different modes of PS externalization by lipid scramblases. Under basal resting conditions, the amino-phospholipids (PE and PS) are restricted to the inner surface of the plasma membrane (A). During conditions of cell stress or platelet activation (B) or during apoptosis (C), membrane asymmetry is lost and PS is externalized to the extrafacial surface (depicted in red). Under basal conditions (A), plasma membrane asymmetry is maintained by the combined activity of cellular flippases and floppases. During cell stress or during platelet activation, intracellular calcium levels rise, resulting in the activation of TMEM16F, and PS exposure to the extracellular leaflet (B). During apoptosis and the activation of caspases, executioner caspases are able to cleave and activate Xkr8, as well as cleave and inactivate ATP11C, resulting in PS exposure to the extracellular leaflet. Since PS externalized PS via TMEM16F and Xkr8/ATP11C are differentially recognized as eat-me signals, it is likely that the density of PS, or the oxidation state of the PS, provide assurance signals for efferocytosis. Key: PE, phosphatidylethanolamine; PC, phosphatidylcholine; PI, phosphatidylinositol; PS, phosphatidylserine; Oxidized PS, oxidized phosphatidylserine.