Bacterial genomes and plasmids encode a variety of peptide and protein toxins that mediate inter-bacterial competition. Bacteriocins are diffusible proteins that parasitize cell-envelope proteins to enter and kill bacteria. Contact-dependent growth inhibition (CDI) is one mechanism of inter-bacterial competition. Novel Toxin 21 (alternatively 16S rRNA endonuclease CdiA) belongs to a family of prokaryotic polymorphic toxin systems implicated in intra-specific conflicts. This RNase toxin found in bacterial polymorphic toxin systems, is proposed to adopt the BECR (Barnase-EndoU-ColicinE5/D-RelE) fold, with two conserved lysine residues and [DS]xDxxxH, RxG[ST] and RxxD motifs. In bacterial polymorphic toxin systems, the toxin is usually exported by the type 2, type 4, type 5 or type 7 secretion systems. This is also referred to as the E. cloacae CdiAC. The CdiAC proteins carry a variety of sequence-diverse C-terminal domains, which represent a collection of distinct toxins. Many CdiA-CT toxins have nuclease activities. In accord with the structural homology, CdiA-CT cleaves 16S rRNA at the same site as colicin E3 and this nuclease activity is responsible for growth inhibition.