Of particular interest, inactivation of the X‐linked CDKL5 gene, or disease‐associated mutations commonly found in the N‐terminal CDKL5 catalytic domain, causes a neurodevelopmental disorder termed CDKL5 deficiency disorder (CDD; Tao et al, 2004; Weaving et al, 2004), which has some overlapping features with Rett syndrome (Scala et al, 2005) and West syndrome (Kalscheuer et al, 2003).§ Here, CDKL5 is linked to atypical Rett syndrome.