Fusion assay between ASCT2 -transduced and syncytin-Ory1 -transduced co-cultured cells demonstrates that ASCT2 is the syncytin-Ory1 receptor. Left panel: Cell-cell fusion was assayed upon independent transfections of a set of A23 cells with an empty vector (none) or an expression vector for either the syncytin-Ory1, syncytin-1 or syncytin-2 protein together with an nls-LacZ gene-expression vector, and another set of A23 cells with an expression vector for the syncytin-1 receptor ASCT2, the syncytin-2 receptor MFSD2  or an empty vector (none). One day after transfection, cells were resuspended and pairs of transfected cells from each set were cocultured for 1-2 days, fixed and X-Gal stained. Right panel: Syncytia can be easily detected (arrows) for the syncytin-Ory1/ASCT2, syncytin-1/ASCT2 and syncytin-2/MFSD2 pairs, with only mononucleated cells visible in the other cases. Abbreviations: syn-Ory1, syncytin-Ory1; syn1, syncytin-1; syn2, syncytin-2.