Comparison of cDNA-derived MR766 and FSS13025 virus growth. Vero cells (2.5 × 10 5 /well) in 12-well plates were co-transfected with 1 µg pTight-ZIKV/MR766 or pTight-ZIKV/FSS13025 DNA and 1 µg pTet-On vector, followed by the addition of 1 µg/mL Doxycycline to cell culture media. Infectious virus titers in the supernatants collected at day 3 (MR766) or day 6 (FSS13025) were quantified by a plaque assay. ( A ) Plaque formation by cDNA-derived ZIKV/MR766 and ZIKV/FSS13025 viruses. Vero cells in 12-well cell culture plate were infected with 30 PFU of either MR766 or FSS13025. Plaques were stained and visualized after 4 days (for MR766) or 6 days (for FSS13025) p.i. ( B ) Growth curves of cDNA-derived MR766 and FSS13025 viruses. To compare the growth ability between MR766 and FSS13025, 4 × 10 5 Vero cells seeded in 6-well plates were infected with MR766 or FSS13025 virus at 0.01 MOI at 37 °C for 1 h. Upon washing with phosphate-buffered saline (PBS) three times, infected Vero cells were incubated with 2 mL DMEM containing 10% FBS. At day 1, 2, and 3 post-infection, virus in the supernatant was collected and stored at −80 °C. Virus yields at different time points were determined by a plaque assay. Data points represent the mean titer of triplicates. Statistical significance was analyzed by Student's t -test: ** p