Evolution of CRISPR-Cas systems resulted in incredible structural and functional diversity. Evolution of CRISPR-Cas systems resulted in incredible structural and functional diversity. Class 1 CRISPR-Cas systems are considered to be the evolutionary ancestral systems. The class 2 systems have evolved from class 1 systems via the insertion of transposable elements encoding various nucleases, and are now being used as tools for genome editing. Prarthana Mohanraju et al. Science 2016;353:aad5147 Copyright © 2016, American Association for the Advancement of Science