HDAC9 and plasma cell myeloma: Recently, histone deacetylase (HDAC) inhibitors have been reported to have anti-tumor effects, and some HDAC inhibitors, such as vorinostat, romidepsin, and panobinostat, have been approved by the United States Food and Drug Administration (FDA) for treating peripheral T cell lymphoma and multiple myeloma [11,12,13,14].