Fusogenic activity of syncytin-Ory1. (A) Assay for cell-cell fusion mediated by syncytin-Ory1. The indicated cell lines were transfected with an expression vector for syncytin-Ory1 or an empty vector (none) together with a LacZ expression vector. Cells were cultured for 1-2 days after transfection, fixed and X-gal-stained. Syncytia (arrows) were detected in syncytin-Ory1-transfected SH-SY5Y cells, with only mononucleated cells visible in the other cases. (B) Assay for cell infection mediated by syncytin-Ory1-pseudotyped virus particles. Pseudotypes were produced by cotransfection of human 293T cells with expression vectors for the SIV core, the syncytin-Ory1 protein (or an empty vector) and a LacZ-containing retroviral transcript. Supernatants were used to infect the indicated target cells, which were X-gal stained 3 days after infection. Abbreviation: Syn-Ory1, syncytin-Ory1.