Replication of SIVmac055 in tenofovir-treated and untreated macaques infected with SIVmac239Δnef. Macaques chronically infected with SIVmac239Δnef were treated for 4 weeks with tenofovir (1488, 1514) or left untreated (1494, 1512). Both treated and untreated macaques were challenged with SIVmac055 on day 28 (arrow). Replication of SIV was measured by allele-specific PCR to discriminate between SIVmac055 (●) and SIVmac239Δnef (○). Data is expressed as SIV RNA copies/ml of plasma.