Tumor formation and reduced transcription of both sFRP1 gene and sFRP2 gene were found in Dlg gene knock-out mice, and thereby the following has been provided: an agent for enhancing the expression and/or function of sFRP, containing a compound having an effect of enhancing the expression and/or function of Dlg; an agent for inhibiting tumor formation or an agent for preventing and/or treating a tumor disease, containing the agent for enhancing the expression and/or function of sFRP; a method of enhancing the expression and/or function of sFRP, comprising enhancing the expression and/or function of Dlg; a method of inhibiting tumor formation or a method of preventing and/or treating a tumor disease, comprising using the aforementioned enhancing agent or the aforementioned enhancing method; a non-human mammal that is deficient in one or both of Dlg alleles; a cell originating in the mammal; a method of identifying a compound, comprising using the mammal or the cell; and a method of examining a tumor tissue or a tumor cell, comprising measuring the expression and/or function of Dlg.