CD202b,also known as Tie-2 or TEK, is a 145 kD type I transmembrane protein. It is a member of the receptor tyrosine kinase (RTK) family of proteins and is expressed by endothelial cells and their progenitors, quiescent hematopoietic stem cells (HSCs), and a subpopulation of monocytes. Angiopoietin-1 (Ang-1) is an activator of CD202b to promote, maintain, and stabilize mature vessels and to maintain HSCs in the quiescent state. Ang-2 is another ligand of CD202b, which is involved in postnatal angiogenesis and in antagonizing the effects of Ang-1. Tie-2 also binds to Ang-4.