Maps of defense arsenals in microbial genomes To survive the attack of foreign invaders such as viruses and plasmids, bacteria and archaea fight back with immune systems that are usually clustered in “defense islands” in their genomes. Doron et al. took advantage of this property to map microbial defense systems systematically (see the Perspective by Kim). Candidate immune systems were then experimentally validated for their activities. Like well-known defense arsenals such as restriction-modification and CRISPR systems, these additional immune systems now require mechanistic investigation and could potentially be engineered into useful molecular tools in the future. Science , this issue p. eaar4120 ; see also p. 993