SUMO1 and Xeroderma pigmentosum complementation group C: Different UBL domains recognize different substrates: UBL1-2 has the binding sites of ring finger protein (RNF) 168 and 169 (the negative regulator of RNF168), xeroderma pigmentosum complementation group C (XPC), DNA methyltransferases 1 (DNMT1), and ICP0 (10–14).