1. H. M. Foss , C. J. Roberts , K. M. Claeys , E. U. Selker , Science 262 , 1737 ( 1993 ).
2. A. Hagemann J. Irelan H. Foss C. Roberts E. Selker unpublished data.
3. A. Hagemann and E. Selker unpublished data.
4. 'Present address: Department of Radiation Oncology
5. University of Washington Medical Center Seattle WA