HDAC9 and neoplasm: The acetyl-histone H3 (Ac-H3) and acetyl-histone H4 (Ac-H4) levels in these primary tumor tissues confirmed a stronger inhibitory effect of ZN444B on histone deacetylases compared with SAHA, which confirmed the HDAC inhibitory effect of ZN444B on breast cancer in vivo (Fig. 3E, F).