A lac promotor mutant was constructed by filling in the protruding ends of the HpaII site located within the lac promotor. The mutation, named M42, is a two base pair insertion that changes the distance between the -10 and -35 regions from 18 to 20 residues. The activity of the mutant promotor measured in vivo is 15% of the wild-type promotor. The M42 promotor is sensitive to the catabolite repression in the manner similar to that of the wild type. Sequences of several deletions within the lac promotor are also given.