Despite the promising clinical results produced by HDAC inhibitors in treatment of haematological malignancies such as T-cell lymphoma, no apparent clinical evidence indicates that HDAC inhibitors work effectively as a monotherapy against solid tumours including breast cancer.19,37,38 Here, we demonstrated that HDAC5 promotes the growth of TAMR cells and decreases the overall survival of breast cancer patients with tamoxifen treatment, which makes HDAC5 a potential therapeutic target for the reverse of tamoxifen resistance. This evidence concerns the gene HDAC5 and breast cancer.