CD135 is a tyrosine kinase receptor expressed on normal cells including CD34+ hematopoietic stem cells, myelomonocytic progenitors, primitive B cell progenitors, and thymocytes. CD135 is also expressed on malignant hematopoietic cells including AML, ALL and CML BC. CD135, also known as FMS-like tyrosine kinase 3, FLT3, STK1, and Flk2, is a growth factor receptor that binds the FLT3 ligand to promote the growth and differentiation of primitive hematopoietic cells. The intracytoplasmic domain of CD135 is modified by phosphorylation and has been shown to interact with Grb2, SOCS1, VAV1, and Shc. In humans, expression of Flt3 is restricted to subsets of CD34 positive as well as CD34 negative normal bone marrow cells. In these cells, the level of expression of Flt3 is rather low. Most of the CD34 bright Flt3+ cells co-express CD117 at high levels. They may represent early cycling, but not quiescent stem cells. Flt3+ cells in the CD34lo and CD34- populations do not co-express CD117 molecule and may represent B lymphoid precursors.