CXCR4 cell surface expression. The CXCR4 coding sequence was mutated in the pcDNA3.1 plasmid, cloned into pBABE-puro and finally expressed in GHOST.CD4 cells. Cell surface expression was monitored by flow cytometry using phycoerythrin labeled anti-CXCR4 mAb 12G5. Coreceptor expression is expressed as % of CXCR4 wt expression of GHOST.CD4.CXCR4 cells. Error bars are calculated based on three independent experiments.