To explore the molecular basis of PPP3CB downregulation, we first considered an epigenetic mechanism as we showed that a class I histone deacetylase (HDAC) inhibitor, entinostat (SNDX-275 or MS-275) mainly inhibited HDAC1 and potently induced apoptosis in HER2-positive breast cancer cells33. The gene discussed is HDAC9; the disease is breast cancer.