1. Institute of Experimental Hematology, Hannover Medical School, 30625 Hannover, Germany.
2. Institute of Virology, Campus Benjamin Franklin, Charité Medical School, 12203 Berlin, Germany.
3. Molecular Virology and Gene Delivery Section, Laboratory of Biochemical Genetics, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.