[1] Univ Salamanca, Dept Med & Transgen Facil, Salamanca 37007, Spain
[2] Inst Biol Mol & Celular Canc CSIC USAL, Salamanca 37007, Spain
[3] Univ Salamanca, Dept Fisiol & Farmacol, Salamanca 37007, Spain
[4] Univ Tokyo, Inst Mol & Cellular Biosci, Lab Chromosome Dynam, Tokyo, Japan
[5] Ctr Invest Biol CSIC, Madrid 28040, Spain
[6] Cornell Univ, Ctr Vertebrate Genom, Ithaca, NY 14850 USA