1. J. J. Peschon, J. L. Slack, P. Reddy, K. L. Stocking, B. J. Castner, R. S. Johnson, J. N. Fitzner, N. Nelson, C. J. Kozlosky, M. F. Wolfson, C. T. Rauch, D. P. Cerretti, R. J. Paxton, C. J. March, and R. A. Black are with Immunex Corporation, 51 University Street, Seattle, WA 98101, USA. S. W. Sunnarborg and D. C. Lee are in the Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, NC 27599–7260, USA. W. E. Russell is in the Departments of Pediatrics and Cell Biology,...