The difference between Tat/dCA affinity (ITC, 100 nM) and dCA inhibitory activity in cell-based assays (0.6 to 2 nM) (Table 2) (10) could be the result of the nature of the Tat-TAR feedback loop that amplifies activity in vivo; especially since the amount of Tat in vivo after infection is very low (24). The gene discussed is TAT; the disease is infection.