1. D. E. Cressman, L. E. Greenbaum, R. A. DeAngelis, R. Taub, Department of Genetics and Medicine, University of Pennsylvania School of Medicine, 705a Stellar-Chance, 422 Curie Boulevard, Philadelphia, PA 19104-6145, USA.
2. G. Ciliberto and V. Poli, Instituto di Ricerche di Biologia Molecolare, IRBM Pietro Angeletti, Via Pontina Km 30.600, Pomezia (Roma), Italy.
3. E. E. Furth, Department of Pathology, University of Pennsylvania School of Medicine, 705a Stellar-Chance, 422 Curie Boulevard, Philadelphia, PA 19104-6145, USA.