Despite clinical efficacy in patients with cutaneous T-cell lymphomas, the therapeutic window of the currently available HDAC inhibitors may not suffice for meaningful anti-tumour efficacy in breast cancer when used as a single agent without more careful patient selection or the definition of a biomarker (Luu et al, 2008). The gene discussed is HDAC9; the disease is primary cutaneous T-cell non-Hodgkin lymphoma.