Extensive efforts over the past 20 years have produced dozens of HDAC inhibitors, of which four HDAC inhibitors (voinostat, romidessin, panabinostat, and belinostat) have been clinically approved for the treatment of cutaneous and peripheral T-cell lymphomas as well as multiple myeloma, and has been tested in several other cancers [97]. Here, HDAC9 is linked to AL amyloidosis.