Replication of HPV16 or HPV18 genomes supported by homologous or heterologous E1/E2s. 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 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 when cells were transfected 20 ng of each pF16E1 and pF16E2 (A) or 10 ng of each pF18E1 and pF18E2 (B). Each bar represents the average of two independent experiments with the standard error of mean.