Interleukin 3 （IL-3） is a protein that in humans is encoded by the IL3 gene localized on chromosome 5q31.1. Sometimes also called colony-stimulating factor, multi-CSF, mast cell growth factor, MULTI-CSF, MCGF; MGC79398, MGC79399: the protein contains 152 amino acids and its molecular weight is 17 kDa. IL-3 is produced as a monomer by activated T cells, monocytes/macrophages and stroma cells. The major function of IL-3 cytokine is to regulate the concentrations of various blood-cell types. It induces proliferation and differentiation in both early pluripotent stem cells and committed progenitors. It also has many more specific effects like the regeneration of platelets and potentially aids in early antibody isotype switching.