Simultaneous replication of HPV16/18 genomes in the presence of E1/E2s of both types. C33A cells were transfected with a mixture of HPV16 and HPV18 gDNAs (1 ng each) together with the indicated amounts (ng) of the expression plasmids. Three days after transfection, low molecular weight DNA was isolated by the Hirt procedure and digested with Dpn I. The Dpn I-resistant HPV16 (upper panel) and HPV18 (lower panel) gDNAs were 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 with the homologous E1/E2 alone. Each bar represents the average of three independent experiments with the standard deviation.