1. The Laboratory of Molecular Biology and Department of Genetics, University of Wisconsin, Madison, Wisconsin 53706, U.S.A.; Division of Neurosciences, City of Hope Research Institute, 1450 East Duarte Road, Duarte, California 91010, U.S.A.
2. The Laboratory of Molecular Biology and Department of Genetics, University of Wisconsin, Madison, Wisconsin 53706, U.S.A.; Department of Zoology, The University of Iowa, Iowa City, Iowa 52242, U.S.A.
3. The Laboratory of Molecular Biology and Department of Genetics, University of WisconsinMadison, Wisconsin 53706, U.S.A.