1. V. Papayannopoulos, V. M. Panin, C. Rauskolb, K. D. Irvine, Waksman Institute and Department of Molecular Biology and Biochemistry, Rutgers, The State University, Piscataway, NJ 08854, USA. A. Tomlinson, Department of Genetics and Development and Center for Neurobiology and Behavior, College of Physicians and Surgeons of Columbia University, New York, NY 10032, USA.