A Programmable Dual-RNA–Guided DNA Endonuclease in Adaptive Bacterial Immunity
This study demonstrated that the CRISPR-associated protein Cas9 from Streptococcus pyogenes is an RNA-guided DNA endonuclease whose target specificity is determined by a dual-RNA structure formed by a CRISPR RNA (crRNA) base-paired to a trans-activating crRNA (tracrRNA). The authors showed that Cas9 introduces site-specific double-strand breaks in target DNA, with its HNH domain cleaving the complementary strand and its RuvC-like domain cleaving the noncomplementary strand. Critically, they engineered the two guide RNAs into a single chimeric guide RNA that still directed sequence-specific cleavage, establishing the system as a programmable tool for genome editing.