EPI-X4 (hSA408–423 peptide) is an antagonist for C-X-C motif chemokine receptor 4 ( CXCR4 ) with IC 50 of 8.6 μM. EPI-X4 blocks the CXCL12-mediated signaling, inhibits chemokine-mediated migration and invasion of leukemia cell. EPI-X4 exhibits anti-inflammatory activity in mouse model. EPI-X4 exhibits antiviral activity against CXCR4-tropic HIV with IC 50 of 8.6 μM.