This family includes the Smr (Small MutS Related) proteins, and the C-terminal region of the MutS2 protein. It has been suggested that this domain interacts with the MutS1 protein in the case of Smr proteins and with the N-terminal MutS related region of MutS2. This domain exhibits nicking endonuclease activity that might have a role in mismatch repair or genetic recombination. It shows no significant double strand cleavage or exonuclease activity. The full-length human NEDD4-binding protein 2 also has the polynucleotide kinase activity.