1. F. Fang, G. Orend, E. Ruoslahti, La Jolla Cancer Research Foundation, Cancer Center, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA.
2. N. Watanabe and T. Hunter, Molecular Biology and Virology Laboratory, Salk Institute for Biological Studies, La Jolla, CA 92037, USA.