[1] Univ Cambridge, Dept Pathol, Mammary Apoptosis & Dev Grp, Cambridge CB2 1QP, England
[2] Ludwig Inst Canc Res, Cell Signalling Lab, London W1W 7BS, England
[3] Roslin Inst, Roslin EH25 9PS, Midlothian, Scotland
[4] Univ Tokyo, Fac Med, Dept Internal Med 3, Tokyo 113, Japan
[5] UCL, Dept Biochem & Mol Biol, London WC1E 6BT, England