TF and infection: In contrast, following infection with the 776-2E-SM1 mutant, viral loads remained at about 10 SV40 DNA copies per cell in both cell lines at 28 d.p.i. In the TF-1a cells, this difference between the levels of WT and SM1 viral DNA was significant (p < 0.05) at 17 d.p.i. and thereafter.