Francesco Muntoni1, J Helen Cross2. 1. Developmental Neuroscience Programme, UCL Institute of Child Health and Great Ormond Street Hospital for Children, London WC1N 1EH, UK. Electronic address: f.muntoni@ucl.ac.uk. 2. Developmental Neuroscience Programme, UCL Institute of Child Health and Great Ormond Street Hospital for Children, London WC1N 1EH, UK.