SET domains are protein lysine methyltransferase enzymes. SET domains appear to be protein-protein interaction domains. A subset of SET domains have been called PR domains. The SET domain consists of two regions known as N-SET and SET-C. SET-C forms an unusual and conserved knot-like structure of probably functional importance. Additionally to SET-N and SET-C, an insert region (SET-I) and flanking regions of high structural variability form part of the overall structure. This domain is found in fungi associated with SET and N-SET domains.