Several lac repressor mutants have been isolated which repress beta-galactosidase synthesis in Escherichia coli up to 200-fold. They do so by binding specifically to particular symmetrical lac Oc operator variants. The mutations in the lac repressor are localized in two separate parts of the recognition helix comprising (i) residues 1 and 2 which interact with base pairs 4 and 5 of lac operator and (ii) residue 6 which recognizes operator base pair 6. Mutations of residues 1 and 2 may be combined with a mutation of residue 6. The resulting mutant protein binds specifically to an operator variant with three symmetric exchanges in base pairs 4, 5 and 6.