1. Instituto de Fisiología Celular y Depto de Bioquimica, Facultad de Medicina, Universidad Nacional Autónoma de México, Apartado postal 70-242, Mexico 04510, D.F., Mexico
2. Escuela de Biología, Universidad Autónoma de Puebla, Puebla, Mexico
3. Department of Paediatrics, Imperial College School of Medicine at St Mary's, London W2 1PG, UK
4. Krebs Institute for Biomolecular Research, Department of Molecular Biology and Biotechnology, The University of Sheffield, Sheffield S10 2TN, UK