Axl, also known as Ufo and Ark, is a widely expressed glycoprotein in the TAM receptor tyrosine kinase family. TAM family receptors (Dtk/Tyro3, Axl, and Mer) are involved in regulation of the inflammatory response, cell survival and migration, and tumorigenesis. Axl binds the vitamin K-dependent protein Gas6, triggering platelet aggregation and thrombus formation, macrophage and dendritic cell phagocytosis of apoptotic cells, NK cell development from hematopoietic progenitor cells, and in vivo angiogenesis. Axl is highly expressed in solid cancers and promotes in vivo tumorigenesis and tumor cell invasiveness. It contributes to vascular remodeling and inflammatory cell infiltration in response to hypertension and restricted blood flow. It also functions as a cellular entry receptor for Gas6-opsonized lentiviruses.