Effects of heterologous E1 or E2 on HPV16/18 replication. C33A cells were transfected with 1 ng of the HPV16 (A) or HPV18 (B) gDNAs together with the indicated amounts (ng) of the expression plasmids for E1/E2s. Three days after transfection, low molecular weight DNA was isolated by the Hirt procedure and digested with Dpn I. The Dpn I-resistant HPV gDNA was quantified by real-time PCR and normalized to that of the luciferase gene. The level of the replication was presented as the relative amount of the Dpn I-resistant DNA compared to that obtained by the replication with the homologous E1/E2 alone. Each bar represents the average of two independent experiments with the standard error of mean.