HDAC9 and Anxiety: A study testing functional redundance of class I HDACs (HDAC1 and HDAC2) observed Hdac2 but not Hdac1 knockout mice – while not exhibiting a change in locomotor and anxiety-related behavior – to display enhanced performance in an attentional set-shifting task along with accelerated conditioned fear extinction [120], which is in line with enhanced fear extinction elicited by pharmacological HDAC inhibition as detailed above (3.3.2).