HDAC9 and primary cutaneous T-cell non-Hodgkin lymphoma: Four HDAC inhibitors including SAHA, FK228, FXD101, and LBH589 are clinically approved by the US FDA for the treatment of cutaneous T-cell lymphoma and multiple myeloma, and a HDAC inhibitor, HBI-8000 is approved by the Chinese FDA for the treatment of cutaneous T-cell lymphoma (Fig. 1)16–20.