Figure 3(a, b).

Live cell two color fusion assay and immune-neutralization of cell fusion using K type human endogenous retrovirus envelope monoclonal antibodies. Two color fusion assay (a and b). A375 cells stably expressing pS-puro-scrambled or pS-puro-H-Ki were labeled with blue Hoechst 33342, and A375 cells stably expressing pEYFP-N3 were labeled with green BODIFY using CellTracker probes for long-term tracing of living cells reagents according to manufacturer's instruction (Invitrogen, Carlsbad, CA). After dye labeling, 107 Hoechst 33342-labeled pS-puro-scrambled or pS-puro-H-Ki cells were each mixed with 10⁷ green BODIFY-labeled pEYFP-N3 expressing cells. Mixed cells were cultured for 12 h without puromycin or G418 selection. Cells were then cultured in medium containing both puromycin and G418 for 48 h to select for puro-neo fused green (Green BODIFY) and blue (Hoechst 33342) double stained cells. Upper left, blue-green double colored cells were present when pEYFP-N3 expressing cells were mixed with pS-puro-scrambled (a), but blocked with pS-puro-H-Ki expressing cells (b). Lower left and right, blue and green stained cells, respectively. Photographed by fluorescence microscope. Upper right, photographed through phase contrast microscope