CXCR4 and infection: Additionally, the infection efficiency of the dual tropic 89.6 HIV-1 in both U87.CD4.CXCR4 and U87.CD4.CCR5 parental cells was much lower than other strains of HIV-1 and this was particularly the case for infection of HIV-1 89.6 in U87.CD4.CCR5 cells, which was approximately 2~3-fold above the control background.