In addition, the SCFA propionic acid produced by A. muciniphila activated the cell cycle inhibitor p21 through G protein-coupled receptor 43 (GPR43) and downregulated inhibitor of apoptosis protein (IAP), which restrained cancer cell proliferation, induced apoptosis, and improved the antitumor effect of ICI [87] (Fig. 4A). The gene discussed is FFAR2; the disease is cancer.