Bacterial endoribonuclease toxins in the toxin-antitoxin systems. The types of toxin-antitoxin systems are determined by how antitoxins antagonize the endoribonuclease toxins. In type I toxin-antitoxin systems, antitoxins are antisense RNAs and inhibit toxin translation by base-pairing. In type II toxin-antitoxin systems, antitoxins are proteins that directly bind to toxin and neutralize toxin’s endoribonuclease activity. Anti-toxin alone or toxin-antitoxin complex also binds to the promoter region and represses expression of the toxin-antitoxin operon. In type III toxin-antitoxin systems, antitoxins are small noncoding RNAs that are transcribed as longer transcripts and then processed by the cognate endoribonuclease toxin into 34-36 nt sRNAs. The processed sRNAs bind to the toxin protein and inhibit toxin’s endoribonuclease activity.