1. Department of Biomedical Sciences, Sheffield City Polytechnic, Pond Street, Sheffield S1 1WB, U.K.
2. Wayne State School of Medicine, Detroit, MI 48201, U.S.A.
3. University of Texas Health Science Center, Houston, TX 77225, U.S.A.
4. University of Michigan Medical School, Ann Arbor, MI 48109, U.S.A.