By monitoring the abundance of acetylated histones (H4ac, H4K12ac, H4K16ac, and H3K18ac) in human cancer cell lines and embryonic stem cells, McBrian et al. [113] demonstrated that as the internal pH decreases, histones are globally deacetylated by HDAC and the released acetate anions are co-exported with protons outside the cells preventing further reduction in intracellular pH, but with consequences in the expression of numerous genes. Here, HDAC9 is linked to cancer.