The purpose of this protocol was to determine whether plerixafor in combination with Granulocyte Colony Stimulating Factor (G-CSF) can increase the circulating levels of peripheral blood stem cells (PBSCs) in patients whose peripheral CD34+ counts remain low after treatment with G-CSF alone, whether it was safe, and whether transplantation with the apheresis product was successful, as measured by time to engraftment of polymorphonuclear leukocytes (PMNs) and platelets (PLTs).