Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) is a powerful growth and differentiation factor which acts on hematopoietic progenitor cells and also activates differentiated granulocytes and macrophages. The 22 kDa glycosylated GM_x005f_x001E_CSF, similar to IL3 and IL5, is a cytokine with a core of four bundled alpha -helices. Mature human GMCSF shares 63%70% amino acid sequence identity with canine, feline, porcine, and rat GMCSF and 54% with mouse GMCSF. GM-CSF has been and is still widely used as an adjuvant in clinical trials of vaccination with autologous tumor cells, peptides and/or dendritic cells in a variety of human neoplasms. The effects of GM-CSF on HIV-1 replication in primary monocytes and macrophages, and in related cell lines, have demonstrated augmentation of HIV-1 expression in vitro.