Defects in STK9, a dual-specificity serine/threonine kinase, are a cause of atypical Rett syndrome. Rett syndrome is an X-linked dominant disease. It is a progressive neurologic developmental disorder and one of the most common causes of mental retardation in females. Patients appear to develop normally until 6 to 18 months of age, then gradually lose speech and purposeful hand movements and develop microcephaly, seizures, autism, ataxia, intermittent hyperventilation, and stereotypic hand movements. After initial regression, the condition stabilizes and patients usually survive into adulthood. Rett syndrome due to CDKL5-associated mutations is characterized by a severe early-onset phenotype and atypical features such as infantile spasms.