HDAC9 and cancer: The boronic acid group is attacked by water in the active site, forming a transition state analogue that binds to the zinc ion, leading to the inhibition of HDAC that subsequently lead to cancer cell growth inhibition (59, IC50 of 2.0 μM; 60, IC50 of 2.0 μM; 61, IC50 of 1.4 μM).