The PDGF family of growth factors consists of five different disulphide-linked dimers built up of four different polypeptide chains encoded by four different genes. These five isoforms act via two receptor tyrosine kinases, PDGF receptors alpha and beta (RTK class III, PDGF receptor family). The PDGFs have a common structure with the typical growth factor domain involved in the dimerization of the two subunits, and in receptor binding and activation. All four PDGF chains contain a highly conserved growth factor domain, denoted the PDGF/VEGF homology domain. Upon activation by their endogenous ligands, these receptors dimerize, and are activated by auto-phosphorylation of several sites on their cytosolic domains, which serve to mediate binding of co-factors and subsequently activate signal transduction, for example, through the PI3K and the MAPK pathways. Both PDGF and VEGF family members are potent mitogenic and angiogenic factors with critical roles in tumor formation as well as embryonic development and wound healing.FLT3 (Fms-liketyrosine kinase 3, aka CD135) is a cytokine receptor which belongs to the class III receptor tyrosine kinase family. It is expressed on the surface of many hematopoietic progenitor cells. Notably, approximately one-third of acute myeloid leukemia (AML) patients have mutations of this gene, and such mutations are one of the most frequently identified types of genetic alterations in AML. The majority of the mutations involve an internal tandem duplication (ITD) in the juxtamembrane (JM) domain of FLT3, which is specifically found in AML.