HDAC9 and mature T-cell and NK-cell non-Hodgkin lymphoma: Romidepsin is a potent, class 1 selective histone deacetylase (HDAC) inhibitor 1–3 approved by the US Food and Drug Administration (FDA) for the treatment of patients with cutaneous or peripheral T-cell lymphoma (CTCL, PTCL) who received ≥1 prior systemic therapy 4.