CDKL5 and atypical Rett syndrome: Mutations of the gene encoding methyl-CpG-binding-domain-containing-protein 2 (MECP2) are the underlying cause for the vast majority of people affected with typical Rett syndrome (RTT; OMIN #312750) (Amir et al., 1999), with mutations of the genes encoding cyclin-dependent kinase-like 5 (CDKL5) and forkhead box g1 (FOXG1) also causal for other less common RTT cases (Weaving et al., 2004; Ariani et al., 2008).