Filgrastim is a synthetic form of GCSF produced in E.coli.[6] PBSC donors are given a course of GCSF prior to PBSC collection, this ensures a better outcome, as stem cell proliferation increases, thus increasing the number of peripheral stem cells in circulation. The course is usually given over a 4-day period prior to PBSC collection.[7] Mild bone pain usually results due to the excessive stem cell crowding within the bone marrow.