[1] Univ Cambridge, Wellcome Trust Canc Res UK CRUK Gurdon Inst, Cambridge CB2 1QN, England
[2] Univ Cambridge, Dept Biochem, Cambridge CB2 1QN, England
[3] CNRS, Inst Chim Subst Nat, F-91198 Gif Sur Yvette, France
[4] Wellcome Trust Sanger Inst, Cambridge CB10 1SA, England