1. Center for Cancer Research, National Cancer Institute at Frederick, Frederick, Maryland 21702, Gilead Sciences and IOCB Research Centre, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo n. 2, Prague, Czech Republic, Department of Biochemistry, Faculty of Natural Science, Charles University, Hlavova 2030, Prague, Czech Republic, and MGI Pharma, Inc., 6611 Tributary Street, Baltimore, Maryland 21224