This is a nuclease (NUC) domain found in Cpf1, an RNA-guided endonuclease of a type V CRISPR-Cas system. Structural and functional analysis indicate that this domain is involved in DNA cleavage [1].

1: Crystal Structure of Cpf1 in Complex with Guide RNA and Target DNA. Yamano T, Nishimasu H, Zetsche B, Hirano H, Slaymaker IM, Li Y, Fedorova I, Nakane T, Makarova KS, Koonin EV, Ishitani R, Zhang F, Nureki O; Cell. 2016;165:949-962. PMID:27114038