[1] Univ Birmingham, Dept Paediat & Child Hlth, Sect Med & Mol Genet, Birmingham B15 2TT, W Midlands, England
[2] Univ Cambridge, Dept Pathol, Cambridge CB2 1QP, England
[3] Univ Cambridge, Dept Anat, Lab Stem Cell Biol, Cambridge, England
[4] Babraham Inst, Lab Dev Genet & Imprinting, Cambridge, England
[5] Genos Biosci, La Jolla, CA 92037 USA