We explored the evolution of CCR5 and CXCR4 use following 5P12-RANTES selection by evaluating single cycle infection mediated by CC1/85 env clones isolated after 13 (round 1), 25 (round 2), 30 (round 3), and 36–44 (round 4) weeks of virus replication (Fig. 3). The gene discussed is CXCR4; the disease is infection.