SHH and Dravet syndrome: Regarding the causes of Shh signaling impairment in DS, recent data suggest that the triplicated gene APP (amyloid precursor protein), a gene that is important for cell cycle progression and neuron migration (Nalivaeva and Turner, 2013), may be a key candidate underlying trisomy-linked alteration of Shh signaling (Trazzi et al., 2013).