HeLa and 293T cells maintained in our lab [44] were cultured in DMEM containing 10% fetal bovine serum. Purified anti-E 4E11 and 4G2 monoclonal antibodies were provided by Dr. A. Amara and P. Despres (Institut Pasteur, France), respectively. Purified anti-DV1 mouse IgG and sera from four patients infected by the four dengue serotypes (1) (2) (3) (4) , respectively, were kindly provided by Dr. Philippe Buchy (Institut Pasteur, Cambodia). The native (non-codon optimized) DV1 (strain FGA/NA d1d) prME gene containing construct was provided by Dr A. Amara. The prME sequences of DV1 (strain FGA/NA d1d), DV2 (strain FGA/02), DV3 (strain PAH881/88) and DV4 (strain 63632) were codon-optimized and synthesized by the Geneart Company (Regensburg, Germany) and subcloned into pcDNA or retroviral vector pCHMWS-IRES-Hygromycin (kindly provided by Dr. Rik Gijsbers, from Molecular Medicine at Katholieke Universiteit Leuven) using BamHI and XhoI restriction sites. A nucleotide sequence encoding for the signal peptide of VSV-G (MKCLLY-LAFLFIGVNC) was included upstream of each prME gene. Sequences of codon-optimized prME genes are provided as Supporting Information ( Figure S1 ).