[1] Genentech Inc, Dept Prot Engn, San Francisco, CA 94080 USA
[2] Mt Sinai Hosp, Samuel Lunenfeld Res Inst, Toronto, ON M5S 1A8, Canada
[3] Univ Toronto, Dept Biochem, Toronto, ON M5G 1L6, Canada
[4] Univ Toronto, Banting & Best Dept Med Res, Toronto, ON M5G 1L6, Canada
[5] Univ Toronto, Dept Mol & Med Genet, Toronto, ON M5G 1L6, Canada