Four HDAC inhibitors (HDACis), vorinostat (SAHA), belinostat (PXD101), panobinostat (LBH589), and romidepsin (FK228), have been approved by the US FDA for the treatment of cutaneous T-cell lymphoma (CTCL), relapsed or refractory peripheral T-cell lymphoma (PTCL), and multiple myeloma (MM), whereas chidamide (CS055) has also been approved by China’s NMPA for the treatment of relapsed or refractory peripheral T-cell lymphoma (249). This evidence concerns the gene HDAC9 and peripheral T-cell lymphoma, not otherwise specified.