CD309 is also known as vascular endothelial growth factor receptor 2 (VEGFR2) and fetal liver kinase-1 (Flk-1). CD309 is a member of the tyrosine protein kinase family that contains a single pass transmembrane receptor with a protein kinase domain and seven immunoglobulin-like domains in the extracellular region. CD309 is expressed at high levels in adult heart, lung, kidney, brain, and skeletal muscle. It's a receptor for VEGF or VEGFC, and plays an important role in the development of vascular endothelial cells, hematopoietic cells, and vascular permeability.