Targeted pharmacological inhibition of the CXCR4/SDF-1α interaction with plerixafor resulting in rapid mobilization of HSCs and other CXCR4-expressing cells from the BM into the circulation (33–37) and the observation that aberrant CXCR4 expression on tumor cells results in preferential BM metastasis (34, 38) highlight the singular importance that CXCR4 plays in cellular homing to the BM. The gene discussed is CXCL12; the disease is neoplasm.