[1] HARVARD UNIV, SCH MED, CTR BLOOD RES, BOSTON, MA 02215 USA
[2] HARVARD UNIV, SCH MED, DEPT PATHOL, BOSTON, MA 02215 USA
[3] MIT, DEPT BIOL, CTR CANC RES, HOWARD HUGHES MED INST, CAMBRIDGE, MA 02139 USA
[4] GENENTECH INC, DEPT IMMUNOL, San Francisco, CA 94080 USA