A. Four to six week-old Rag1−/− mice were inoculated subcutaneously with ZIKV-Dakar 41525 (ZIKV-Dak) (3 experiments, n = 8). B. Scheme of in vivo passaging of ZIKV-Dak. Brain homogenates (106 PFU) from Rag1−/− mice that succumbed to infection were inoculated subcutaneously into naive Rag1−/− mice. C. Four to five week-old WT C57BL/6 mice were treated with anti-Ifnar1 mAb one day prior to subcutaneous inoculation with 105 FFU of ZIKV-Dak or ZIKV-Dak-MA (3 experiments, n = 15, log rank test; ***, P < 0.001). D–F. ZIKV RNA levels in serum (D), spleen (E), and brain (F) of WT mice after treatment with anti-Ifnar1 mAb and subcutaneous inoculation of ZIKV-Dak or ZIKV-Dak-MA (3 experiments, n = 8–12, Mann-Whitney test; *, P < 0.05). Solid lines are median values, and dotted lines denote the limit of detection of the assay. G. RNA was isolated from ZIKV-Dak-MA and subjected to next generation sequencing. Mutations present in over 95% of genomes are shown. See also Table S1 and Fig S1.