A homology domain of unclear function, occurs in the C-terminal region of severalregulatory components of the 26S proteasome as well as in other proteins. This domainhas also been called the PINT motif (Proteasome, Int-6, Nip-1 and TRIP-15) [<cite idref="PUB00005480"/>].Apparently, all of the characterised proteins containing PCI domains are parts of largermulti-protein complexes. Proteins with PCI domains include budding yeast proteasomeregulatory components Rpn3(Sun2), Rpn5, Rpn6, Rpn7and Rpn9 [<cite idref="PUB00005950"/>]; mammalian proteasome regulatory components p55, p58 and p44.5, and translationinitiation factor 3 complex subunits p110 and INT6 [<cite idref="PUB00005773"/>, <cite idref="PUB00005799"/>]; ArabidopsisCOP9 and FUS6/COP11 [<cite idref="PUB00005762"/>]; mammalian G-protein pathway suppressor GPS1, and severaluncharacterised ORFs from plant, nematodes and mammals. The complete homology domain comprisesapprox. 200 residues, the highest conservation is found in the C-terminal half. Several of theproteins mentioned above have no detectable homology to the N-terminal half of the domain.