A plaque variant of SV40 that was first isolated in the 1960s, designated SV40-LP(KT), was molecularly cloned and subjected to sequence analysis. The genome of SV40-LP(KT) was found to be nearly identical to the previously described isolate known as 777. However, SV40-LP(KT) contained a mutation in the VP1 coding region resulting in a change of histidine 136 to tyrosine. This VP1 mutation was identified as a genetic determinant influencing a number of phenotypes associated with SV40-LP(KT) such as plaque morphology, intracellular vacuole formation, and ganglioside receptor usage.