STXBP1 and epilepsy: Two additional examples of epilepsy where cognitive impairment and developmental consequences are not clearly correlated with the severity and characteristics of seizures are provided by the diseases associated to the mutations of PCDH19 and STXBP1. Mutations of the PCDH19 gene in female patients causes early onset epilepsy, in part resembling DS (Dibbens et al., 2008; Specchio et al., 2011), and often lead to cognitive phenotypes ranging from borderline to severe intellectual disability (Depienne et al., 2009; Hynes et al., 2010; Marini et al., 2010).