This family contains MPN (also known as Mov34, PAD-1, JAMM, JAB, MPN+) domains variants lacking key residues in the JAB1/MPN/Mov34 metalloenzyme (JAMM) motif and are unable to coordinate a metal ion. Comparisons of key catalytic and metal binding residues explain why the MPN-containing proteins Rpn7/PSMD7, Rpn8/PSMD8, CSN6, Prp8p, and the translation initiation factor 3 subunits f and h do not show catalytic isopeptidase activity. It has been proposed that the MPN domain in these proteins has a primarily structural function. Rpn7 is known to be critical for the integrity of the 26S proteasome complex by establishing a correct lid structure. It is necessary for the incorporation/anchoring of Rpn3 and Rpn12 to the lid and essential for viability and normal mitosis. CSN6 is a highly conserved protein complex with diverse functions, including several important intracellular pathways such as the ubiquitin/proteasome system, DNA repair, cell cycle, developmental changes, and some aspects of immune responses. It cleaves ubiquitin-like protein Nedd8 (neural precursor cell expressed, developmentally downregulated 8)) in the cullin 1 in cells. EIF3f s a potent inhibitor of HIV-1 replication as well as an important negative regulator of cell growth and proliferation. EIF3h regulates cell growth and viability, and that over-expression of the gene may provide growth advantage to prostate, breast, and liver cancer cells.