1. Wellcome/CRC Institute of Cancer and Developmental Biology, Cambridge CB2 1QR, U.K.
2. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, U.S.A.
3. Health and Environment Unit, Laval University Medical Research Center, CHUQ, Québec, Canada G1V 4G2
4. Department of Medical Biology, Faculty of Medicine, Laval University, Québec, Canada G1K 7P4