1. Differentiation Programme, EMBL, Posffach 10.2209, Heidelberg 69012, Germany
2. Laboratory of Eukaryotic Molecular Genetics, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, U.K.
3. Laboratory of Cellular Immunology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, U.K.