CRISPR is a term for Clustered Regularly Interspaced Short Palidromic Repeats. A number of protein families appear only in association with these repeats and are designated Cas (CRISPR associated) proteins. The family describes Cas proteins of about 400 residues that include the motif [VIL]-D-x-[ST]-H-[GS]. The CRISPR and associated proteins are thought to be involved in the evolution of host resistance. The exact molecular function of this family is currently unknown.

1: A guild of 45 CRISPR-associated (Cas) protein families and multiple CRISPR/Cas subtypes exist in prokaryotic genomes. Haft DH, Selengut J, Mongodin EF, Nelson KE; PLoS Comput Biol. 2005;1:e60. PMID:16292354