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The mutation of Apc is an important early genetic event in colon carcinogenesis. However, it remains to be clarified what kinds of cooperative genes are required for complete carcinogenesis. To identify cooperative genes for the ApcMin mutation the authors carried out retroviral insertional mutagenesis (RIM) using Min mouse-derived IMCE colon epithelial cells. Anchorage-independent transformed colonies were induced by retroviral infection only in IMCE cells, while no transformation was found in young adult mouse colon (YAMC) cells that are normal for Apc. One hundred and fifty-seven retroviral integration sites (RIS) were identified in 101 independent transformants, and four common integration sites (CIS), Dnah3, Ahnak, Stk17b and Rbm9, were observed. Upregulation of Dnah3 and Ahnak, and truncation of Dnah3 due to the viral integration, was revealed. In addition, Dnah3-overexpressing IMCE cells showed impairment of microtubule function. These data suggest the importance of cytoskeletal function in Apc-related tumor development and the usefulness of RIM in non-hematopoietic tissues, providing new insight into the early stage of colon carcinogenesis. (Cancer Sci 2008; 99: 979–985)