A polynucleotide comprising a nucleotide sequence encoding Vif, wherein each of the amino acids corresponding to positions 127, 128, 130, 131, 132 and 142 of the amino acid sequence in FIG. 1A representing the Vif wild-type consensus sequence, are replaced with another amino acid, and wherein the nucleotide sequence does not encode the amino acid sequence shown in FIG. 2, representing the Vif sequence of the F12 non-producer variant of HIV-1.