Data from InterRett was seminal to the delineation of CDKL5 deficiency disorder (CDD; OMIM 300203) as a clinically distinct disorder since research by our group found less than one quarter of individuals with a pathogenic CDKL5 variant met the criteria for the early-onset seizure variant RTT [15]. This evidence concerns the gene CDKL5 and craniodiaphyseal dysplasia.