HDAC9 and breast carcinoma: Preclinical data has shown that valproic acid inhibits the majority of class I and II HDAC at CI50, varying from 0.7–1.3 mM [24]; however, the concentration needed in vitro to achieve its biological effects varies from system to system, but in some cases the concentration required is lower than 0.5 mM in tumors such as glioma, hematopoietic, or breast cancer cell lines [24-26].