The 26S proteasome is a key component of the ubiquitin-proteasome system,a process responsible for the majority of cellular protein degradation. hRpn13 (also termed ADRM1 or GP110) is a novel 46-kDa subunit of its 19S regulatory complex. hRpn13 binds directly to the proteasome-associated deubiquitinating enzyme,UCH37,and enhances its isopeptidase activity. Overexpression of hRpn13 promotes the activity of the ubiquitin-proteasome system and modulates the influence of osteoblasts on osteoclasts by controlling the stability of regulatory proteins in osteoblasts.