[1] Univ Penn, Dept Cell & Dev Biol, Philadelphia, PA 19104 USA
[2] Gurdon Inst, Cambridge CB2 1QN, England
[3] Dept Pathol, Cambridge CB2 1QN, England
[4] Wistar Inst Anat & Biol, Philadelphia, PA 19104 USA
[5] Stowers Inst Med Res, Kansas City, MO 64110 USA