The different mutants of HIV-1 IRES were made by PCR amplification with four primers. The external primers were Ext-KpnI(+) et Ext-BamHI(-). The details for all the primers used can be found in Supplementary Table 1 . The resulting PCR products were cloned in the KpnI and BamHI restriction sites of pFRT-dual-IRES-HIV.