NFKB1 and neoplasm: Interestingly, a preferred expression in the tumor compared to healthy tissue was observed for the constitutively active cytomegalovirus promoter (CMV) when using low vector doses: while enhanced vector uptake in the tumor due to leaky vessels likely contributes to this finding, an at the first glance unexpected effect, the enhanced activity of Nuclear Factor kappa B (NF-κB) in tumors, which binds and activates the CMV promoter, is proposed to play a major role [43].