In 2003, Kalscheuer [1] identified the locus of the gene, encoding a serine-threonine kinase (STK9) on the X chromosome (Xp22.3), and suggested the involvement of this gene mutation in the pathogenesis of X-linked infantile spasms (ISSX) based on the analysis of two cases of female patients with infantile spasms and profound mental retardation. The gene discussed is CDKL5; the disease is infantile spasms.