Coffin-Lowry syndrome (CLS, MIM#303600) is an X-linked semidominant disorder. In males disorder is characterized by severe mental retardation with distinctive phenotype of face and hands, and with abnormalities in osteoarticular system. In females the intensity of symptoms is variable. Coffin- Lowry syndrome is caused in the majority of cases by mutations in the RSK2 gene (RPS6KA3) located in Xp22.2 region. The RSK2 gene encodes for RSK2 protein that belongs to a family of serine-threonine kinases acting in the MAPK/ERK signalling pathway.