Axl belongs to the TAM family of receptor tyrosine kinases, which consists of Tyro3, Axl, and Mer. All three family members have similar structures and share a number of ligands, including the vitamin K–dependent ligands growth arrest protein 6 (Gas6) and protein S. In normal tissues, TAM receptor tyrosine kinases contribute to immune response regulation, including clearance of apoptotic cells and inhibition of cytotoxic immune activation in response to apoptosis. When cells undergo apoptosis, the polarity of the plasma membrane lipid bilayer is altered, externalizing the anionic phospholipid phosphatidylserine (PS).
