CRISPR/Cas9 is an RNA-guided DNA-targeting system derived from the type II CRISPR adaptive immune systems of bacteria. Its core components are the Cas9 endonuclease and a guide RNA, which may consist of separate CRISPR RNA (crRNA) and trans-activating crRNA (tracrRNA) molecules or a fused single-guide RNA (sgRNA) ^[1]. Base pairing between the guide-derived spacer sequence and a complementary DNA protospacer directs Cas9 to a specified genomic or extrachromosomal locus. Target recognition additionally requires a nearby protospacer-adjacent motif (PAM), whose sequence is determined by the particular Cas9 ortholog; for the commonly used Streptococcus pyogenes Cas9, this motif is generally 5′-NGG-3′ ^[1][2]. After PAM recognition and guide–target hybridization, the HNH and RuvC nuclease domains of Cas9 cleave opposite DNA strands to generate a double-strand break ^[1][3]. CRISPR/Cas9 therefore denotes the guide-RNA-programmable Cas9 system itself, rather than the broader CRISPR family, which includes mechanistically distinct class 1 and class 2 CRISPR systems and non-Cas9 effector proteins ^[3].