The PspCas13b entry in FlyBase represents an RNA-guided ribonuclease which is derived from the naturally occurring type VI-B ribonuclease encoded by the Prevotella sp. P5-125 Cas13b gene and in which the two HEPN ribonuclease domains present in the naturally occurring protein are functional. The exact sequence of the nuclease may differ depending on the particular transgene or modified endogenous locus being used, for example, the coding sequence may have been codon optimized for use in different species and a nuclear localization signal may have been added. Cas13b ribonucleases specifically target single-stranded RNA for cleavage. They are able to process a precursor CRISPR repeat array into mature guide RNAs (gRNAs), each of which contains a spacer sequence complementary to the target RNA, plus additional sequence required for binding to Cas13b. In the presence of the mature gRNA, Cas13b recognises and binds the complementary target RNA. This forms an active ternary complex in which the two HEPN ribonuclease domains of Cas13b act together to cleave the complementary target RNA (PMID:28065598). These properties mean that PspCas13b can potentially be used to target specific RNAs of interest for knockdown, by providing either an unprocessed guide RNA (pre-gRNA) array or a mature gRNA that targets the RNA of interest; use of a pre-gRNA array would allow multiple gRNAs that target different sequences of the same RNA or that target multiple RNAs to be supplied simultaneously. However, at least in vitro, activated Cas13b-containing ternary complexes also show cleavage of collateral or 'bystander' RNAs (that are not complementary to the gRNA), potentially limiting the use of PspCas13b for specific RNA targeting. For a comparison of PspCas13b with other CRISPR-based RNA-targeting systems, see PMID:29940185 and PMID:30388409.