HDAC9 and cancer: There are currently several phase I clinical trials exploring the combination regimen of HDAC inhibitors to achieve better cancer treatment230,231 and many novel HDAC inhibitors are undergoing phase I and II clinical trials.232,233 In addition, SCFAs may directly affect the transcription of HDAC genes.118 In GPR43-deficient mice, cycle adenosine monophosphate (cAMP)-protein kinase A (PKA)-cAMP response element binding (CREB) pathway is enhanced, resulting in the overexpression of HDAC.