Organization of the genomic loci that encode prokaryotic immune systems including toxin genes. The core genes of CRISPR-Cas, RM, and DND systems in predicted operons are shown by pink arrows; genes with (predicted) toxin activity are shown by different colors, and the (predicted) toxin domains are denoted with red outline. The Csa3 protein in the Type IA system lacks the HEPN domain. Abbreviations: HEPN - higher eukarytoes and prokaryotes nucleotide-binding domain; Sir2, ParB and REase, DEDD are nucleases from distinct superfamilies. A. CRISPR-Cas. Gene names follow the nomenclature and classification from . B. Restriction-modification. Gene names follow the nomenclature and classification from . C. Phosphorothioation. Gene names follow the nomenclature from . D. Prokaryotic argonaut, pAgo.