1. David R. Cheriton School of Computer Science, University of Waterloo, Waterloo N2L 3G1, Canada
2. Program in Cell Biology and SPARC BioCentre, Hospital for Sick Children, 686 Bay St, Toronto, Ontario M5G 0A4, Canada
3. Department of Molecular Genetics, University of Toronto, 686 Bay St, Toronto, Ontario M5G 0A4, Canada