Highly selective CXCR4 chemokine receptor antagonist (IC50 values are 0.02 - 0.13 and > 25 μM for CXCR4 and all other chemokine receptors respectively). Switches inflammatory responses from Th2 to Th1 type and reduces airway hyperresponsiveness in a mouse model of asthma. Potently inhibits HIV-1 and HIV-2 replication in vitro (EC50 = 4 - 35 nM) and mobilizes hematopoietic stem cells in vivo.