Abstract Ras associated with diabetes (Rad) is a Ras-related GTPase that promotes cell growth by accelerating cell cycle transitions. Rad knockdown induced cell cycle arrest and premature senescence without additional cellular stress in multiple cancer cell lines, indicating that Rad expression might be critical for the cell cycle in these cells. To investigate the precise function of Rad in this process, we used human Rad as bait in a yeast two-hybrid screening system and sought Rad-interacting proteins. We identified the Grap2 and cyclin D interacting protein (GCIP)/DIP1/CCNDBP1/HHM, a cell cycle–inhibitory molecule, as a binding partner of Rad. Further analyses revealed that Rad binds directly to GCIP in vitro and coimmunoprecipitates with GCIP from cell lysates. Rad translocates GCIP from the nucleus to the cytoplasm, thereby inhibiting the tumor suppressor activity of GCIP, which occurs in the nucleus. Furthermore, in the presence of Rad, GCIP loses its ability to reduce retinoblastoma phosphorylation and inhibit cyclin D1 activity. The function of Rad in transformation is also evidenced by increased telomerase activity and colony formation according to Rad expression level. In vivo tumorigenesis analyses revealed that tumors derived from Rad knockdown cells were significantly smaller than those from control cells (P = 0.0131) and the preestablished tumors are reduced in size after the injection of siRad (P = 0.0064). Therefore, we propose for the first time that Rad may promote carcinogenesis at least in part by inhibiting GCIP-mediated tumor suppression. Cancer Res; 70(11); 4357–65. ©2010 AACR.