The famed physicist-turned-biologist, Max Delbrück, once remarked that, for physicists, “the field of bacterial viruses is a fine playground for serious children who ask ambitious questions.” Early discoveries in that playground helped establish molecular genetics, and half a century later, biologists delving into the same field have ushered in the era of precision genome engineering. The focus has of course shifted—from bacterial viruses and their mechanisms of infection to the bacterial hosts and their mechanisms of immunity—but it is the very same evolutionary arms race that continues to awe and inspire researchers worldwide. In this review, we explore the remarkable diversity of CRISPR–Cas adaptive immune systems, describe the molecular components that mediate nucleic acid targeting, and outline the use of these RNA-guided machines for biotechnology applications. CRISPR–Cas research has yielded far more than just Cas9-based genome-editing tools, and the wide-reaching, innovative impacts of this fascinating biological playground are sure to be felt for years to come.
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