Effects of the non synonymous SNPs identified in envFRD and envW on their fusogenic function. A, Construction of the env-expression vectors and rationale of the fusion assay. Each of the env allelic forms were PCR amplified from genomic DNA and cloned into the phCMV expression vector. Cells were transfected with the env-expression vectors and stained with May-Grünwald and Giemsa solutions. B, Cell-cell fusion assay for the allelic forms of envFRD (upper panel) and envW (lower panel), using two cells types (human 293T and TE671 cells). The fusion index represents the percentage of fusion events in the transfected cell populations as evidenced by syncytia formation, and is quantitated as in . The control corresponds to transfection of an expression vector without an env gene. The most frequent haplotypes correspond to FRD359A/367T and W138R/307S.