AXL and infection: Other entry factors that facilitate attachment or entry include neuropilin-1 [40, 41], the tyrosine-protein kinase receptor UFO (AXL) [42], CD147 [43], high-density lipoprotein (HDL) scavenger receptor B type 1 (SR-B1) [44], integrins [45, 46], angiotension II receptor 1 (AT1) and vasopressin receptor 2, but their role in natural infection is currently unclear.