F2 and infection: In order to address the same question for SV40 Pt mutant, that is whether constitutive expression of SV40 LT-Ag is sufficient to bring the replication level of SV40 Pt mutant closer to that of SV40 WT, Cos-7 cells which constitutively express SV40 LT-Ag, were transfected with either SV40 (776) WT or its Pt mutant as described in the legend for Figure 9, and the efficiency of infection was examined by both immunocytochemistry (Figure 9A) and Western blotting (Figure 9B).