S100B and infection: Infections were carried out with viruses produced from the chimeric proviral clones NL4, 3-luc-1558 and NL4, 3-luc-HXB that encode the luciferase reporter gene in place of nef. The env gene in these chimeric proviral constructs was subcloned respectively from the primary QH1558 and the laboratory-adapted HXB3 isolates, both of which are X4-tropic viruses [15].