Discordant HIV DNA synthesis and R5 versus X4 virus replication in child CD4⁺ T-cell lines. Four independent CD4⁺ T-cell lines from pediatric donors were infected with R5 and X4DNase-treated viruses, and RT activity was detected in culture supernatant up to day 15 (top panel). The levels of HIV-1 gag DNA were measured between 6 and 168 hours after infection (bottom panel). Mean ± SEM of the 4 independent experiment are shown. Whereas only R5 virus replication was detected by RT activity, HIV DNA synthesis occurred with comparable efficiency after both R5 and X4 infection up to 48 hours after which R5 viruses showed a superior capacity to spread in culture.