PTHS is associated with enlarged ventricles, cerebellar atrophy, and hippocampal and corpus callosum hypoplasia (Peippo et al., 2006; Amiel et al., 2007; Zweier et al., 2008; Goodspeed et al., 2018; Zollino et al., 2019), suggesting that gross brain development is sensitive to dramatic changes in TCF4 expression and function. The gene discussed is TCF4; the disease is Pitt-Hopkins syndrome.