1. Lawrence Berkeley National Laboratory, Genomics Division, 1 Cyclotron Road MS64-121, Berkeley, CA 94720, USA, 2Ontario Institute for Cancer Research, MaRS Centre, South Tower, 101 College Street, Suite 800, Toronto, ON, Canada M5G 0A3, 3Department of Genetics, University of Cambridge, Downing Street, Cambridge CB2 3EH, UK and 4BIO5 Institute, The University of Arizona, Tucson, AZ 85719, USA