1. DNA Enzymology and Molecular Virology, Istituto di Genetica Molecolare,IGM-CNR, National Research Council, via Abbiategrasso 207, I-27100 Pavia,Italy
2. Institute of Veterinary Biochemistry and Molecular Biology, University of Zürich-Irchel, Winterthurerstrasse 190, CH-8057 Zürich,Switzerland