HDAC9 and breast cancer: Chidamide (an oral HDAC inhibitor with subtype specificity for the inhibition of HDAC1, HDAC2, HDAC3, and HDAC10) combined with exemestane (AI) improved the progression-free survival in advanced HR+ HER2- breast cancer patients who progressed after a previous endocrine therapy compared with placebo plus exemestane (42).