-!- The first type of this enzyme found proved to be the protein cyclophilin, which binds the immunosuppressant cyclosporin A. -!- Other distinct families of the enzyme exist, one being FK-506 binding proteins (FKBP) and another that includes parvulin from Escherichia coli. -!- The three families are structurally unrelated and can be distinguished by being inhibited by cyclosporin A, FK-506 and 5-hydroxy-1,4-naphthoquinone, respectively.

-!- The RING domain of E3 ubiquitin transferase serves as a mediator bringing the ubiquitin-charged E2 ubiquitin-conjugating enzyme and the acceptor protein together to enable the direct transfer of ubiquitin through the formation of an isopeptide bond between the C-terminal glycine residue of ubiquitin an the epsilon-amino group of an L-lysine residue of the acceptor protein. -!- The RING-E3 domain does not form a catalytic thioester intermediate with ubiquitin (unlike the HECT domain, EC 2.3.2.26). -!- RING-type ubiquitin transferases may occur as single-chain enzymes but also in dimeric forms or in multi-subunit assemblies. -!- Formerly EC 6.3.2.19 and EC 6.3.2.21.