HDAC9 and neuroblastoma: One of the HDAC inhibitors, valproic acid (VPA), inhibits growth and induces differentiation of human neuroblastoma UKF-NB-2 and UKF-NB-3 cells in vitro at concentrations ranging from 0.5 to 2 mM that have been achieved in human with no significant adverse effects (Cinatl et al., 1996).