Prokaryotic enzymes for viral defense The arms race between prokaryotes and viruses provides a strong evolutionary force to create diverse enzymatic activities that mediate antiviral immune responses. These immune components often cluster together in the host genomes, leading to expanded defense systems. Taking advantage of the evolutionary modularity of defense systems, Gao et al. bioinformatically predicted defense genes in most available bacterial and archaeal genomes. In addition, they reconstituted the newly identified systems in Escherichia coli and verified their defense functions against specific bacteriophages. In particular, they characterized defense functions for several predicted nucleoside triphosphatases. Science , this issue p. 1077