HDAC9 and cancer: However, most of them are pan-HDAC (SAHA, LBH-589) or class I selective (FK-228, PXD-101) inhibitors, which usually lead to several mild to severe side effects.16,18,19 In addition, most of HDAC inhibitors lack visible efficacy against solid tumor,14,20 the doses given in clinical are much higher, which severely limit their clinical utility for the treatment of broad spectrum of cancer.