Prokaryotes and their viruses constitute a vastly diverse and highly abundant group of biological entities on earth. During the course of evolution, prokaryotes have developed numerous defensive strategies against invading genetic elements. Viruses, in turn, have evolved counteracting mechanisms. CRISPR-Cas constitutes the only known prokaryotic adaptive immune system against invading viruses and plasmids. Researchers from the University of Copenhagen have characterised an archaeal virus-encoded anti-CRISPR protein and their results are now being published in Cell.