CD135 / FLT3, also known as FLK2 or STK-1 is a receptor tyrosine kinase that plays important roles in hematopoiesis. After binding of Flt3 ligand (FL), CD135 homodimerizes and stimulates proliferation, differentiation and protects the cell from apoptosis. The loss of CD90 and gain of CD135 expression marks the loss of self-renewal in hematopoietic stem cell population. Detectable CD135 expression appears first at low levels on the surface of primitive multilineage progenitor cells and disappears during defined stages of B-cell development, but is upregulated and maintained during maturation of monocytes. CD135 is also expressed on thymocytes, dendritic cell progenitors and on mature dendritic cells, as well as on various malignant hematopoietic cells.