ARX and Cornelia de Lange syndrome: Our methodological approach could be used to understand the natural history and undertake genotype/phenotype characterisation in other RNDDs such as Aristaless-related homeobox gene (ARX) spectrum disorders [36], Cornelia de Lange syndrome [37], Sotos syndrome [38], Pallister–Killian syndrome [39], and Rubinstein–Taybi syndrome [40], where to our knowledge no such databases have yet been established.