HDAC9 and mature T-cell and NK-cell non-Hodgkin lymphoma: Romidepsin, a structurally unique, potent, bicyclic class I selective HDAC inhibitor, has demonstrated durable clinical responses in patients with R/R PTCL, leading to its approval by the US Food and Drug Administration (FDA) in 2011 for the treatment of PTCL in patients who have received at least one prior therapy [108].