CDKL5 and craniodiaphyseal dysplasia: CDKL5 (cyclin-dependent kinase-like 5) deficiency disorder (CDD) is a rare and severe X-linked developmental encephalopathy, caused by mutations in the CDKL5 gene [1,2,3], with an estimated incidence of 1:40,000–60,000 live births [4,5] and a female-to-male ratio of 4:1.