[1] Univ Penn, Sch Med, Dept Pathol & Lab Med, Philadelphia, PA 19104 USA
[2] Univ Penn, Sch Med, Dept Med, Philadelphia, PA 19104 USA
[3] Univ British Columbia, Dept Zool, Vancouver, BC V6T 1Z4, Canada
[4] Univ Erlangen Nurnberg, Dept Genet, D-91058 Erlangen, Germany