CD4 and infection: Although Tax is expressed dynamically by infected cells (1, 31), in our experimental setting, Tax expression in T cells after 12 hours in culture strongly correlated with the PVL of sorted CD4+ SP, CD8+ SP, DP, and DN T cells measured by digital droplet PCR (ddPCR; Figure 1E) (32); this allowed us to use Tax expression at 12 hours as a reliable readout for PVL and infection rate.