Romidepsin is a structurally unique, potent, bicyclic class 1 selective histone deacetylase (HDAC) inhibitor that has demonstrated durable clinical responses in patients with relapsed/refractory PTCL, leading to its approval by the US Food and Drug Administration in 2011 for the treatment of PTCL in patients who have received at least one prior therapy. This evidence concerns the gene HDAC9 and mature T-cell and NK-cell non-Hodgkin lymphoma.