CDKL5 deficiency disorder (CDD; OMIM #300672) is a severe neurodevelopmental disease caused by mutations in the cyclin-dependent kinase-like 5 (CDKL5) gene that is characterised by the early onset of drug resistant seizures, strong hypotonia, severe developmental delay, gross motor impairment, prominent autistic features, and some Rett syndrome-like traits (Fehr et al., 2013). Here, CDKL5 is linked to Rett syndrome.