CDKL5 and Down syndrome: Aberrant neuron morphogenesis and synaptogenesis are associated with a number of brain disorders, including fragile-X mental retardation, Rett syndrome, Down syndrome, and CDKL5-related encephalopathy (Belmonte and Allen, 2004; Shepherd and Katz, 2011; Garner and Wetmore, 2012; Ricciardi et al., 2012).