<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 highly conserved core region found in the Cas3 family of proteins. These proteins are found in association with CRISPR repeat elements in a broad range of bacteria and archaea [<cite idref="PUB00009737"/>]. Cas3 is one of four protein families (Cas1 to Cas4) that are associated with CRISPR elements and always occur near a repeat cluster, usually in the order cas3-cas4-cas1-cas2. Cas3 proteins have motifs characteristic of helicases from superfamily 2 and contain a DEAD/DEAH box region and a conserved C-terminal domain. Some but not all Cas3 family members have an N-terminal HD domain region (<db_xref db="INTERPRO" dbkey="IPR006674"/>), although these sequences are not included within this group. These Cas proteins may be involved in DNA metabolism or gene expression [<cite idref="PUB00009737"/>]. </p> Helicase Cas3, CRISPR-associated, core