To evaluate whether Tat entry could have an impact on endothelial cell susceptibility to HIV-1 infection, IC-activated or control HUVEC were exposed to NL4.3, a clade B X4-tropic virus, in the absence or presence of 1 μM (10 μg/mL) of biologically active Tat versus a mutated Tat protein devoid of trans-activating capability (Tatcys22) that was employed as control. This evidence concerns the gene TAT and HIV-1 infection.