HDAC9 and cancer: Furthermore, gut bacteria-derived metabolites are also closely associated with carcinogenesis, cancer progression and anti-cancer therapy, including short-chain fatty acids (SCFAs), secondary bile acids (SBAs), indole derivatives, and trimethylamine-N-oxide (TMAO), etc. Notably, butyrate can inhibit histone deacetylase (HDAC), inducing apoptosis in cancer cells and suppressing their proliferation.