HDAC9 and neoplasm: Possible mechanisms mediating this effect, which require further investigation, include the following: (1) increased concentrations of metabolites, including butyrate or other short-chain fatty acids in tumours, acting via HDAC inhibition or via other pathways; (2) suppression of overgrowth of unfavourable bacteria, such as Parabacteroides genus; and/or (3) enhancement of anti-tumoural immunity.