HDAC9 and mature T-cell and NK-cell non-Hodgkin lymphoma: The epigenetic modifying agent romidepsin is a structurally unique, potent, bicyclic class I selective histone deacetylase (HDAC) inhibitor 9, 10, 11 approved by the US Food and Drug Administration (FDA) for the treatment of cutaneous T‐cell lymphoma in patients who had received ≥1 prior systemic therapy and for the treatment of PTCL in patients who had received ≥1 prior therapy 12.