PDCD1 and colorectal carcinoma: 1. Bile acid sequestrants (e.g., cholestyramine) lower DCA and inhibit tumor growth;2. Fecal DCA concentration and microbial baiF gene (key for DCA synthesis) abundance are potential CRC risk biomarkers;3. Polyamine blockade therapy combined with PD-1 inhibitors may reverse “cold tumor” microenvironment.