Fig. (1) Deduced amino acid sequence of the two HSV TK mutants, mutant 30 and SR39. The top line shows the amino acid sequence of the residues 159-174 of wild-type HSV TK. Two highly conserved tripeptide motifs are boxed and denoted as Sites 3 and 4. The amino acids in bold in the top line denote codons that were targeted for the creation for the randomized libraries and subsequent mutations are shown in the following lines [15,16].