PROBLEM TO BE SOLVED: To provide a molecular targeted drug used for a therapy of gastrointestinal cancer, and to provide a method for selecting gastrointestinal cancer patients who are predicted to respond to the therapy with the molecular targeted drug concerned by revealing the biomarker to predict the therapeutic effect by the molecular targeted drug concerned for every patient.SOLUTION: Using prostaglandin D2 synthetase promotes the endogenous production of prostaglandin D2, and can inhibit the proliferation of gastrointestinal cancer cells. Furthermore, prostaglandin D2 synthetase selectively shows a proliferation inhibition effect to the gastrointestinal cancer cells with high expression level of peroxisomal proliferation factor active receptor γ1 or an mRNA thereof.
