TI - Results Mutagenesis of HSV-1 UL20 . AB - Previously , we reported on the construction and characterization of a panel of 31 mutations within the UL20 gene [30] . These mutations included : . . . . . . . . 1 ) cluster-to-alanine mutants in which a cluster of proximal amino acids were changed to alanine residues ; 2 ) single amino acid replacement mutants within alanine cluster regions ; 3 ) carboxyl terminal truncations of UL20p . Two additional double mutants where constructed for the present study . UL20 mutant CL38 - CL49 combined the two cluster mutations targeting the two putative PHOSphorylation sites in the amino terminus of UL20p . Similarly , the Y38A - Y49A double mutant combined the two specific tyrosine modifications without altering adjacent amino acids . In addition , UL20 mutants CL2 , CL61 , Y38A and Y117A , which were not reported previously , were included in these investigations . All UL20 mutants were tested for their ability to complement UL20-null infectious virus production , as well as either gB or gK -mediated virus-induced cell fusion having the gBsyn3 or gKsyn1 mutation , respectively . The mutated amino acids for each type of mutation included in this study are shown in Table 1 . The constructed UL20 carboxyl terminal truncations are identified with the number of the last remaining amino acid ( ie 204t retains UL20p amino acids 1-204 ) . The location of each mutation with respect to the UL20p topology [30] is shown in Figure 1 .