Tetherin does not affect infectiousness of transferred virus. Tetherinpos or tetherinneg effector cells, infected with wt or Δvpu virus, were co-cultured with tetherinpos or tetherinneg target cells. Ratios of transmission vs transfer were calculated; ratios of Δvpu and wt infections were compared to determine the impact of tetherin and Vpu on viral infectivity. A ratio of one indicates similar viability of the Δvpu and the wt virus. Ratios <1 represent an advantage for Vpu (wt virus), while a ratio >1 indicates a fitness cost of Vpu (Δvpu virus) in viral cell-to-cell spread. Data are derived from three independent experiments; error bars represent SEM.