Coffin-Lowry syndrome is an X-linked mental retardation disorder with dysmorphism caused by mutation of the ribosomal S6 kinase (RSK2) gene. Coffin-Lowry syndrome patients can experience unusual drop episodes whereby an abrupt loss of muscle tone and falling down can be induced by sudden, unexpected tactile or auditory stimuli. We detected a C913T (R305X) mutation in a female Coffin-Lowry syndrome patient with drop episodes. All mutations in our patient and those previously reported in patients with drop episodes result in premature truncation of the RSK2 protein in the N-terminal kinase domain or upstream of this domain.