Cas13

Cas13 is an RNA-guided ribonuclease in the CRISPR-Cas system, belonging to class 2, type VI. It is a programmable RNA-targeting effector found in certain bacteria and archaea. Unlike the more widely known Cas9, Cas13 cleaves RNA rather than DNA, enabling targeted knockdown of transcripts without altering the genome. Cas13 uses a CRISPR RNA (crRNA) to recognize complementary single-stranded RNA sequences and then cleaves the target.

Cas13 contains two HEPN nuclease domains that execute RNA cleavage. Upon recognition of a matching RNA, Cas13 activates cis cleavage of the target and, importantly, engages a collateral, non-specific RNase activity that degrades nearby RNA molecules. This collateral activity underpins certain diagnostic platforms but can be detrimental in living cells if not controlled. Most Cas13 enzymes require minimal sequence constraints near the target; many show no strict PAM requirement, though some subtypes exhibit a short protospacer flanking sequence preference.

Subtypes include Cas13a, Cas13b, Cas13c, and Cas13d (the latter also called CasRx). Cas13d is smaller, facilitating delivery in therapeutic contexts. Variants differ in crRNA length, target range, and collateral activity levels, enabling a range of applications across research and diagnostics.

Applications include research use for RNA knockdown and cellular imaging, as well as diagnostic tools such as SHERLOCK, which use Cas13’s collateral cleavage to produce detectable signals from specific RNA sequences, for example viral RNAs. Cas13-based diagnostics have been developed for infectious diseases and, in some cases, have achieved rapid, sensitive detection in point-of-care settings.

Cas13 was first described in 2016 by Abudayyeh, Gootenberg, and colleagues. Since then it has become a key tool for RNA biology and diagnostics. Challenges include potential off-target activity and delivery considerations for therapeutic use.