While P. aeruginosa induced hyperacetylation of H3K9 and H4K8 in lung epithelium by inhibiting HDAC1/2 activity, Ni2+ caused histone hypoacetylation via increased ROS in hepatoma cells through decreased histone acetyl transferase (HAT) activity [50], indicating differential role of ROS in modulating HAT and/or HDAC activity. Here, HDAC9 is linked to hepatocellular carcinoma.