The promoters used for these vector are usually based on the promoter of the "lac" operon or the T7 promoter, and they are normally regulated by the "lac" operator. These promoters may also be hybrids of different promoters, for example, the Tac-Promoter is a hybrid of "trp" and "lac" promoters. Note that most commonly used "lac" or "lac"-derived promoters are based on the "lac"UV5 mutant which is insensitive to catabolite repression. This mutant allows for expression of protein under the control of the "lac" promoter when the growth medium contains glucose since glucose would inhibit gene expression if wild-type "lac" promoter is used. Presence of glucose nevertheless may still be used to reduce background expression through residual inhibition in some systems.