This domain is found in the Csm1 subunit of the Csm complex found in Thermococcus onnurineus. Csm is a type III-A CRISPR-Cas system, which is an RNA-guided immune defense mechanism that detects and destroys foreign DNA or RNA. This domain is known as domain A and is positioned side by side with domain C. Both domain A and domain C adopt the BABBA topology. Domain A interacts primarily with domain B [1].

1: Crystal structure of the Csm1 subunit of the Csm complex and its single-stranded DNA-specific nuclease activity. Jung TY, An Y, Park KH, Lee MH, Oh BH, Woo E; Structure. 2015;23:782-790. PMID:25773141