 The immunity gene is often produced constitutively, while the BRP is generally produced only as a read-through of the stop codon on the colicin structural gene.The colicin itself is repressed by the SOS response and may be regulated in other ways, as well.Retaining the colicin plasmid is very important for cells that live with their relatives, because if a cell loses the immunity gene, it quickly becomes subject to destruction by circulating colicin.