The Cas6 Crispr family of proteins averaging 140 residues are characterised by having a GhGxxxxxGhG motif, where h indicates a hydrophobic residue, at the C-terminus [1]. The CRISPR-Cas system is possibly a mechanism of defence against invading pathogens and plasmids that functions analogously to the RNA interference (RNAi) systems in eukaryotes [2].

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

2: A putative RNA-interference-based immune system in prokaryotes: computational analysis of the predicted enzymatic machinery, functional analogies with eukaryotic RNAi, and hypothetical mechanisms of action. Makarova KS, Grishin NV, Shabalina SA, Wolf YI, Koonin EV; Biol Direct. 2006;1:7. PMID:16545108