Abstract: Developmental and epileptic encephalopathy 9 (DEE9): The gene PCDH19 (Xq22.1), which encodes the calcium-dependent cell adhesion protein protocadherin-19 (PCDH19), is nowadays considered as one of the most important genes in monogenic epilepsy (Depienne and LeGuern, 2012). Mutations in PCDH19 are responsible for DEE9 (OMIM #300088), a severe neurodevelopmental disorder characterized by early-onset clustering epilepsy, various degrees of cognitive impairment and neuropsychiatric comorbidities, like autism spectrum disorder (ASD) and behavioural problems. DEE9 patients start suffering from seizures around the age of 10 months until adolescence, when seizures tend to reduce or even disappear, while the psychiatric symptoms persist (Depienne and LeGuern, 2012; Kolc et al., 2018).