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 entry, represented by CT1972 from Chlorobaculum tepidum, is found in the CRISPR/Cas subtype Ecoli regions of many bacteria (most of which are mesophiles), and not in Archaea. It is designated Cse1.