Valproic acid (VPA) sodium (2:1) is an orally active HDAC inhibitor, with IC50 in the range of 0.5 and 2 mM, also inhibits HDAC1 (IC50, 400 µM), and induces proteasomal degradation of HDAC2. Valproic acid sodium (2:1) activates Notch1 signaling and inhib