Coffin-Lowry syndrome is caused by mutations in the RSK2 gene (RPS6KA3) (MIM.300075).
References
Nakamura M, Yamagata T, Mori M, Momoi MY. RSK2 gene mutations in Coffin-Lowry syndrome with drop episodes. Brain Dev. 2005 Mar;27(2):114-7. PMID: 15668050
Ausio J, Levin DB, De Amorim GV, Bakker S, Macleod PM. Syndromes of disordered chromatin remodeling. Clin Genet. 2003 Aug;64(2):83-95. PMID: 12859401