Selected gene-neighborhoods of HEPN genes. The gene neighborhood data for some of the genes encoding HEPN domain containing proteins is depicted using arrows. The HEPN gene is marked with an asterisk. The direction of the arrow is the direction of transcription of the gene. The gene name, Genbank identifier (gi), and the species name of the starred gene are shown next to the operon. The multi-gene modules that always co-occur are boxed. The cartoon representations of the genes are not drawn to scale. The depicted operons are typically representative of a types of operons found in a range of diverse organisms. Domain names of most domains follow the Pfam database or literature  (also see Additional file 1). Non standard abbreviations: RM_TRD, restriction-modification target recognition domain.