Some of the latter group have mutations in other genes, such as that of the cyclin-dependent kinase-like 5 (CDKL5), which is described in individuals with an early seizure onset variant of RTT (Kalscheuer et al. 2003) or the forkhead box G1 (FOXG1), which is responsible for the congenital variant of RTT (Ariani et al. 2008). This evidence concerns the gene FOXG1 and Rett syndrome.