This is beta-hairpin domain found in Cas9 proteins from Actinobacteria. The beta-hairpin domain is not conserved in all type II-C Cas9 proteins. The beta-hairpin in Cas 9 from Streptococcus pyogenes blocks the HNH domain active site [1].

1: Structures of Cas9 endonucleases reveal RNA-mediated conformational activation. Jinek M, Jiang F, Taylor DW, Sternberg SH, Kaya E, Ma E, Anders C, Hauer M, Zhou K, Lin S, Kaplan M, Iavarone AT, Charpentier E, Nogales E, Doudna JA; Science. 2014;343:1247997. PMID:24505130