Molecular model of a PEGylated 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. PEGylation is the addition of a polyethylene glycol (PEG) polymer (top left) to a molecule. This improves the solubility of the molecule and helps to 'hide' it from destruction by the immune system.