“The interaction … the (E)-5-(2-bromovinyl)-2′-deoxyuridine deoxyribose ring with the protein is … by substitution of tyrosine 21 and phenylalanine 139 (analagous to herpes simplex virus type 1 histidine 58 and tyrosine 172), which may explain some of the differences in nucleoside sugar selectivity between both enzymes.”