After a negative bone marrow sampling, patients receive sargramostim (GM-CSF) and filgrastim (G-CSF) subcutaneously (SC) once daily on days 1-5 (or G-CSF twice daily alone for 4-5 days). Peripheral blood progenitor cells are collected 24 hours after the last dose of growth factor injection on day 5 and also on day 6, if necessary. The CD34 positive stem cells are then infected by the retroviral mutant MGMT-G156A ex vivo.