English: Some plasmids include a system which kills the cell in case it does not inherit a plasmid during cell division. These toxin-antitoxin systems consist of two or more closely linked genes that encode both a 'poison' and a corresponding 'antidote'. If the plasmid is absent after cell division, the unstable anti-toxin is degraded and the stable toxic protein kills the new cell. This is known as post-segregational killing (PSK). An example is the CcdA/CcdB system which can be found on the F plasmid of Escherichia coli. CcdB targets the DNA gyrase and inhibits partitioning of the chromosomal DNA. As long as the cell contains the F plasmid, CcdB is bound and neutralized by CcdA. Upon loss of the plasmid, CcdA decays and the activity of CcdB kills the cell.