Plerixafor is a bicyclam derivative and is a selective, reversible antagonist of the chemokine receptor CXCR4, blocking it by binding to its cognate ligand, stromal cell factor-1alpha- (SDF-1alpha-), also known as CXCL12. Plerixafor-induced leukocytosis and increased circulating hematopoietic progenitor cells are thought to result from disruption of the binding between CXCR4 and its cognate ligand, which results in the release of both mature and pluripotent cells into the systemic circulation. CD34+ cells mobilized by plerixafor are functional and capable of engraftment, with long-term potential for population reconstitution.