In addition, HDACs act on multiple substrates beyond histones and are involved in the hydrolysis of more than just acetyl groups.[5, 6] The up‐regulation and high expression of different HDAC isoforms are associated with poor prognosis in cancers such as multiple myeloma and acute myeloid leukemia.[7, 8] Therefore, HDAC inhibition is a promising strategy for tumor therapy. Here, HDAC9 is linked to plasma cell myeloma.