The RING finger proteins Mdm2 and MdmX share significant structural and functional similarity and are major antagonists of the tumor suppressor protein p53. Mdm2 is a member of the RING finger family of ubiquitin-protein ligases E3 and targets p53 for proteasome-mediated degradation and, thus, inacitivation. In contrast to Mdm2, MdmX does not have appreciable E3 activity and does not include p53 degradation. In an effort to characterize the E3 activity of Mdm2, we recently obtained evidence that MdmX acts as a stimulatory cofactor of the E3 activity of Mdm2. MdmX binds to Mdm2 and stimulates, for example, Mdm2-mediated ubiquitination of p53 <it>in vitro</it>. In addition, down-regulation fo either MdmX or Mdm2 expression within cells results in p53 accumulation indicating that both Mdm2 and MdmX are actively involved in p53 degradation. In the project the role of MdmX n Mdm2-mediated ubiquitination of proteins will be further characterized by biochemical,molecular and cell biological means <it>in vitro</it> and within cells. Furthermore, factors that interact with MdmX and/or the Mdm2/MdmX complex shall be identified and determined whether they represent ubiquitination substrates or act as regulators of the E3 activity of Mdm2 and/or Mdm2/MdmX complex. The studies will contribute to the elucidation of the mechanisms involved in Mdm2/MdmX-facilitated ubiquitination and the mechanisms by which the activity of this E3 complex is potentially regulated.