1. M. Leitges and S. Stabel, Max-Delbrück-Laboratorium in der Max-Planck-Gesellschaft, Carl-von-Linné-Weg 10, D-50829 Köln, Germany.
2. C. Schmedt, S. Schaal, A. Tarakhovsky, Institute for Genetics, University of Köln, Weyertal 121, D-50931 Köln, Germany.
3. R. Guinamard and J. Davoust, Centre d'Immunologie CNRS-INSERM de Marseille-Luminy, Case 906, 13288 Marseille Cedex 9, France.