Repressor of the lactose operon. Binds allolactose as an inducer. Homotetramer. Removing residues 1-59 results in loss of DNA-binding activity but retains tetrameric structure and inducer-binding activity. Deleting residues 340-360 results in loss of tetramer formation, but retains dimer formation, inducer-binding activity, and DNA-binding activity (if residues 1-59 are present).