CCR5 and infection: Infection of P4-CCR5 and TZM-bl indicator cells[24] with virus stocks containing normalized quantities of p24 antigen derived from 293T cells transiently transfected with the different proviral constructs[24], showed that most VNP- and P-Nefs enhanced virion infectivity, albeit with variable efficacy (Figure 4A, Additional file1: Figure S3).