Mutations in the X-linked cyclin-dependent kinase like 5 (CDKL5, OMIM 300203) gene (previously known as STK9, serine/threonine kinase 9) are responsible for a severe encephalopathy with X-linked infantile spasms (ISSX, OMIM 308350) [68,69,70,71]. This evidence concerns the gene CDKL5 and Encephalopathy.