High Quality Content by WIKIPEDIA articles! A toxin-antitoxin system is a set of two or more closely linked genes that together encode both a protein `poison` and a corresponding `antidote`. When these systems are contained on plasmids – transferable genetic elements – they ensure that only the daughter cells that inherit the plasmid survive after cell division. If the plasmid is absent in a daughter cell, the unstable anti-toxin is degraded and the stable toxic protein kills the new cell; this is known as `post-segregational killing` (PSK). Toxin-antitoxin systems are widely distributed in prokaryotes, and organisms often have them in multiple copies.