Importantly, we show that (1) Cul3-silencing enhances, whereas Cul3-overexpression reduces HIV-1 infection in immortalized and primary cells, (2) the restrictive effect of Cul3 on infection can be assigned to a reduction of viral mRNA expression and (3) the presence of NF-κB/NFAT binding sites within the viral LTR, but not other transcription factor binding sites, are crucial determinants of this effect. The gene discussed is NFKB1; the disease is HIV-1 infection.