Clusters of short DNA repeats with non-homologous spacers, which are found at regular intervals in the genomes of phylogenetically distinct prokaryotic species, comprise a family with recognisable features. This family is known as CRISPR (short for Clustered, Regularly Interspaced Short Palindromic Repeats). A number of protein families appear only in association with these repeats and are designated Cas (CRISPR-Associated) proteins. This minor cas protein is found in at least five prokaryotic genomes: Methanosarcina mazei, Sulfurihydrogenibium azorense, Thermotoga maritima, Carboxydothermus hydrogenoformans, and Dictyoglomus thermophilum, the first of which is archaeal while the rest are bacterial.