1. Institut National de la Santé et de la Recherche Médicale (INSERM) U 429, Hôpital Necker-Enfants Malades, Paris, France.
2. Department of Haematology, Hammersmith Hospital, Du Cane Road, London W1 20NN, UK.
3. Oncology, Haematology Section, University Children's Hospital, Im Neuenheimer Feld 150, D-69120 Heidelberg, Germany.