Antihuman factor IX antibodies after administration of AVC3FIX5 vector. Solid symbols represent titer of rhesus antihuman factor IX IgG determined by microtiter plate immunoassay (ELISA). Open symbols represent antihuman factor IX neutralizing antibodies determined by a Bethesda inhibitor assay. (▪) Low-dose; (•) intermediate-dose; (▴) high-dose.