Granulocyte colony-stimulating factor (G-CSF) is a hematopoietic cytokine that promotes the proliferation and differentiation of cells in the granulocyte lineage. It is produced mainly by monocytes and macrophages upon activation by endotoxin, TNF-α and IFN-γ. Other cells can also secrete G-CSF after IL-1β,TNF-α and LPS activation. In addition, various carcinoma cell lines and myeloblastic leukemia cells can express G-CSF constitutively. G-CSF has a effect in the treatment of leukopenia, AIDS, MDS and bone marrow transplantation. In humans, two distinct cDNA clones for G-CSF, encoding 207 and 204 amino acid precursor proteins, have been isolated. Both protein have identical amino acid sequences except for a three amino acid insertion (deletion) at the 35th amino acid residue from the N-terminus of the mature protein.