1. Unitat de Recerca Biomèdica (D.M.S., O.M.T., J.R., F.M.) Hospital Materno-Infantil Vall d’Hebron, 08035 Barcelona, Spain
2. Departament d’Anatomia Patològica (N.T.), Hospital Materno-Infantil Vall d’Hebron, 08035 Barcelona, Spain
3. Department of Cell Biology, Georgetown University Medical Center (C.A.S.-Q.), Washington, D.C. 20007