Axl, also known as Ufo and Ark, is a widely expressed 140kD 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. Mature mouse Axl consists of a 427aa extracellular domain (ECD) that contains two Ig-like domains and two fibronectin type III domains, a 21aa transmembrane segment, and a 422aa cytoplasmic domain that includes the tyrosine kinase domain. Within the ECD, mouse Axl shares 81% and 95% aa sequence identity with human and rat Axl, respectively. Axl binds the vitamin K dependent protein Gas6 which triggers tyrosine autophosphorylation of the Axl cytoplasmic domain. Activation of Axl induces a broad range of activities including 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. A 70-80kD soluble portion of the Axl ECD can be shed by proteolytic cleavage, and this fragment retains the ability to bind Gas6.