1. Molecular Biology Department, New York State Institute for Basic Research in Developmental Disabilities, Staten Island, New York 10314, CSI/IBR Center for Developmental Neuroscience and Graduate Program in Biology, City University of New York, New York, New York 10016, Department of Human Genetics, Mount Sinai School of Medicine, New York, New York 10029, and Laboratory for Mass Spectrometry and Gaseous Chemistry, The Rockefeller University, New York, New York 10021