1 Program in Molecular Medicine, The Hospital for Sick Children, Toronto, ON M5G 0A4, Canada; Department of Biochemistry, University of Toronto, Toronto, ON M5S 1A8, Canada. Electronic address: forman@sickkids.ca.
2 Department of Structural Biology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA; Department of Microbiology, Immunology and Biochemistry, University of Tennessee Health Sciences Center, Memphis, TN 38163, USA. Electronic address: richard.kriwacki@stjude.org.
3 Department of Molecular Biology and Genetics, HHMI, School of Medicine, Johns Hopkins University, 725 N. Wolfe Street, Baltimore, MD 21205, USA. Electronic address: gseydoux@jhmi.edu.