Cas9

An RNA-guided DNA endonuclease from bacterial CRISPR immune systems, repurposed as the central tool for precision genome editing.

Guide RNA: A designed RNA that pairs with Cas9 and carries the 20-nucleotide sequence matching the DNA target. The programmable component that lets one protein target almost any sequence.
Streptococcus pyogenes: The bacterium whose SpCas9 became the standard editing enzyme, despite being a human pathogen. Source of the widely used SpCas9 variant found in nearly every molecular biology lab.
Single guide RNA: An engineered fusion of crRNA and tracrRNA that lets one RNA molecule direct Cas9 to any target. The design trick from 2012 that turned a two-RNA bacterial system into one lab reagent.
Gene knockout: Disabling a gene by letting NHEJ introduce frameshifting indels at a Cas9 cut site. The simplest and most common use of Cas9 in research laboratories.
Cas12a: A smaller CRISPR nuclease with T-rich PAMs and staggered cuts, known commercially as Cpf1. The main alternative Cas9 within the CRISPR toolbox, with different PAM requirements.
PAM: A short DNA motif, NGG for SpCas9, required adjacent to the target before Cas9 will bind and cut. The license check Cas9 performs before cutting; it defines what can be targeted.
Type II CRISPR system: The CRISPR subtype that uses Cas9, tracrRNA and RNase III to generate mature guide RNAs. Cas9 is the signature nuclease of this single subtype among CRISPR families.
Guide RNA design: Choosing a 20-nucleotide target sequence with the right PAM and minimal predicted off-target matches. The practical craft step every Cas9 experiment starts with.
Genome-wide CRISPR screening: Pooled experiments using thousands of guide RNAs to knock out every gene and find needed ones. Turns Cas9 into a discovery engine for identifying genes behind any measurable trait.
Zinc finger nuclease: An earlier genome editing enzyme targeted by protein-DNA recognition rather than RNA. The protein-engineering approach Cas9 displaced by making retargeting trivial.

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