CRISPR-Cas systems on the offensive

CRISPR-Cas systems are known as bacterial immune systems for protection against phage infections and plasmids. However, many of these mobile genetic elements have obtained partial or complete CRISPR-Cas systems from their hosts and use them for their own purposes e. g. to eliminate competitors. These observations provide impressive insights into the evolution of host-parasite interaction and the subtle border between parasites and symbionts.