<p>Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) are a family of DNA direct repeats separated by regularly sized non-repetitive spacer sequences that are found in most bacterial and archaeal genomes [<cite idref="PUB00043286"/>]. CRISPRs appear to provide acquired resistance against bacteriophages, possibly acting with an RNA interference-like mechanism to inhibit gene functions of invasive DNA elements [<cite idref="PUB00043287"/>, <cite idref="PUB00043288"/>]. Differences in the number and type of spacers between CRISPR repeats correlate with phage sensitivity. It is thought that following phage infection, bacteria integrate new spacers derived from phage genomic sequences, and that the removal or addition of particular spacers modifies the phage-resistance phenotype of the cell. Therefore, the specificity of CRISPRs may be determined by spacer-phage sequence similarity.</p><p>In addition, there are many protein families known as CRISPR-associated sequences (Cas), which are encoded in the vicinity of CRISPR loci [<cite idref="PUB00020781"/>]. CRISPR/cas gene regions can be quite large, with up to 20 different, tandem-arranged cas genes next to a CRISPR cluster or filling the region between two repeat clusters. Cas genes and CRISPRs are found on mobile genetic elements such as plasmids, and have undergone extensive horizontal transfer. Cas proteins are thought to be involved in the propagation and functioning of CRISPRs. Some Cas proteins show similarity to helicases and repair proteins, although the functions of most are unknown. Cas families can be divided into subtypes according to operon organisation and phylogeny. </p><p>This entry represents a family of Cas proteins that are strictly associated with the Cyano subtype of CRISPR/Cas locus and found in several species of Cyanobacteria and several archaeal species. These proteins contain helicase motifs and appear to represent the Cas3 protein of the Cyano subtype of CRISPR/Cas system.</p> CRISPR-associated helicase, CYANO-type