A protein that helps bone marrow make more white blood cells. Filgrastim is also known as GCSF (granulocyte-colony stimulating factor) or by the trade name Neupogen®. It is given to donors who have agreed to donate peripheral blood stem cells. This moves blood stem cells from the bone marrow into the blood stream so that they can be collected by apheresis. It also helps patients increase their white blood cell count after the transplant.