[1] Univ Oxford, John Radcliffe Hosp, Inst Mol Med, Imperial Canc Res Fund Labs, Oxford OX3 9DS, England
[2] Rhone Poulenc Rorer, Ctr Rech Vitry Alfortville, F-94403 Vitry Sur Seine, France
[3] Univ Oxford, John Radcliffe Hosp, Dept Cellular Sci, Oxford OX3 9DU, England