[1] Katholieke Univ Leuven, Dept Human Genet, B-3000 Louvain, Belgium
[2] Katholieke Univ Leuven, Dept Mol & Cell Biol, B-3000 Louvain, Belgium
[3] Li Ka Shing Ctr, Cambridge Res Inst, Canc Res UK, Cambridge CB2 0RE, England
[4] Univ Libre Bruxelles, Expt Med Lab, B-1070 Brussels, Belgium
[5] Univ Cambridge, Addenbrookes Hosp, Cambridge Inst Med Res, Cambridge EB2 0XY, England
[6] Univ Geneva, Fac Med, Dept Genet Med & Dev, CH-1211 Geneva 4, Switzerland