1. Graduate Program in Cellular and Molecular Biology, University of Wisconsin-Madison, 1525 Linden Drive, Madison, WI 53706, U.S.A.
2. The Genome Center of Wisconsin, University of Wisconsin-Madison, 425 Henry Mall, Madison, WI 53706, U.S.A.
3. Department of Biochemistry, University of Wisconsin-Madison, 433 Babcock Drive, Madison, WI 53706, U.S.A.