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. At the same time, colicin is only released from a producing cell by the use of the lysis protein, which results in that cell's death. This suicidal production mechanism would appear to be very costly, except for the fact that it is regulated by the SOS response, which responds to significant DNA damage. In short, colicin production may only occur in terminally ill cells. The Professor Kleanthous Research Group at the University of Oxford study colicins extensively as a model system for characterising and investigating protein-protein interactions and recognition.