Cas9
Pronunciation: kaz-NINE
Also written as: Cas9 — CRISPR-associated protein 9
An RNA-guided endonuclease that creates double-strand breaks at targeted genomic loci; the most widely used nuclease in CRISPR-based editing. Rendered as 'Cas9', not 'CAS9' or 'cas9'.
Full Definition
Cas9 is a CRISPR-associated endonuclease derived from Streptococcus pyogenes (SpCas9) that acts as the 'molecular scissors' of the CRISPR-Cas9 system. It is directed to a specific DNA sequence by a guide RNA (gRNA) and introduces a double-strand break, triggering cellular repair mechanisms. Editors should note the mixed-case convention: 'Cas9', not 'CAS9', 'cas9', or 'Cas-9'. When the organism of origin is specified, the convention is SpCas9 (from S. pyogenes) or SaCas9 (from S. aureus). The gene encoding the protein is italicised in lower case: cas9.
Usage
Usage note: Mixed case is mandatory: 'Cas9'. The gene name is italicised and lowercased: *cas9*. Distinguish from Cas12a and Cas13, which have distinct mechanisms.
In Context
- "The author had written 'CAS9' throughout; this was corrected to 'Cas9' in line with standard nomenclature." — Copy-editing note
- "SpCas9 was delivered as a ribonucleoprotein complex to reduce off-target effects." — Methods section