Molecular model of a granulocyte colony-stimulating factor (G-CSF) molecule. G-CSF is a glycoprotein growth factor that regulates the maturation, proliferation and differentiation of granulocyte white blood cells in the bone marrow. There are three types of granulocytes; neutrophils, eosinophils and basophils. These cells help to destroy invading organisms, and are involved in allergic and inflammatory responses. G-CSF is used clinically to treat neutropenia, a condition characterised by low levels of white blood cells.