Granulocyte colony-stimulating factor (GCSF), molecular model. GCSF (red) is a glycoprotein that acts as a hormone, and is the principal growth factor regulating the maturation, proliferation and differentiation of neutrophilic granulocytes in the bone marrow. This chemical can be used to treat neutropenia (a low concentration of a certain type of white blood cell), particularly as a side-effect of chemotherapy for cancer treatment.