IQGAPs are multidomain scaffolding proteins that integrate Rho GTPase and Ca2+/calmodulin signals with cell adhesive and cytoskeletal reorganizational events. Targeted disruption of the murine Iqgap2 gene resulted in the age-dependent development of apoptosis and hepatocellular carcinoma (HCC), characterized by the overexpression of IQGAP1, the loss of membrane E-cadherin expression, the cytoplasmic translocation (and activation) of beta-catenin, and the overexpression of a nuclear target of beta-catenin, cyclin D1. In normal hepatocytes, IQGAP2 was found to exist as one component of a multifunctional scaffolding complex comprising IQGAP1, beta-catenin, and E-cadherin, with no evidence for direct IQGAP1-IQGAP2 interactions. Interbreeding of Iqgap2(-/-) mice into the Iqgap1(-/-) background resulted in the phenotypic correction of the preexisting hepatopathy, decreases in the incidence and sizes of HCC tumors, and the normalization of overall survival rates compared to those of Iqgap2(-/-) mice, suggesting that maximal penetrance of the Iqgap2(-/-) HCC phenotype requires the coordinate expression of IQGAP1. These results identify Iqgap2 as a novel tumor suppressor gene specifically linked to the development of HCC and the activation of the Wnt/beta-catenin signaling pathway, while also suggesting that IQGAP1 and IQGAP2 retain functionally divergent roles in hepatocellular carcinogenesis.