S100B and infection: We identify HIV-1R7/3 YU-2 Env GFP as a virus, the behavior of which in tg rat T-cells ex vivo displays key characteristics of a spreading, primarily cell-to-cell-mediated infection: first, GFP expression from the nef locus, a surrogate for early viral gene expression in infected cells, increased continuously over periods of 2 weeks with peak levels comparable to human reference cultures.