Bacterial Type II toxin—antitoxin (TA) protein pairs are encoded by adjacent, co-transcribed genes. Most of the toxins belonging to such systems function as endoribonucleases (mRNases), decreasing the half-lives of mRNAs and consequently the global translation rate in bacterial cells, although toxins with other mechanisms of action have also been described. The activity of these proteins is modulated by antitoxins that counteract the growth inhibitory effect of their cognate toxins by direct protein-protein interactions. Toxins are far more stable than their relative antitoxins as the latter are rapidly degraded by intracellular proteases, in response to environmental and physiological stresses (e.g. amino acid starvation).