Transient transfections of CHO cells expressing human HLA-C with different env sequences. CHO (-, grey bars) and CHO-HLA-C (+, black bars) cells transiently transfected with plasmids encoding Tat, Rev and Env from different primary and laboratory HIV-1 isolates and co-cultivated for 6 hours with TZM-bl target cells. After Tat driven transactivation of firefly luciferase expression, fusion efficiency was quantified and expressed as counts per second (CPS). Each value represents the average of four replicates. The gp120/gp41 of primary isolates 93MW965 (R5), 91US005 (R5) and 92UG024 (X4) are HLA-C sensitive (p < 0.05) while isolates J500 (X4) and NDK (X4) are less sensitive to the presence of HLA-C (p not significant).