1. DNAX Research Institute 901 California Avenue Palo Alto California 94304 USA
2. Present address: University of Bern, Länggass‐Strasse 122 CH‐3001 Bern Switzerland
3. Dana–Farber Cancer Institute, Harvard Medical School Boston Massachusetts 02115 USA
4. Department of Experimental Oncology, European Institute of Oncology, Via Ripamonti 435 20141 Milan Italy