To shed light onto the mechanistic link between HDAC and BCSCs, we assessed the effects of the pan-HDAC inhibitors AR-42 and SAHA (vorinostat) versus those of the class I HDAC inhibitor depsipeptide (romidepsin) on mammosphere formation, a surrogate measure of CSC expansion [19, 20], in two breast cancer cell lines, MDA-MB-231 and SUM-159. This evidence concerns the gene HDAC9 and breast carcinoma.