Figure 1. (A) The human DNA topoisomerase I (Top1)–mediated cleavage and religation of DNA. (B) Structure of the norindenoisoquinoline AI-III-52 in a Top1 cleavage complex. The 3´ end of the cleaved strand is covalently linked to the catalytic Tyr residue 723 (Y). (C) An expanded view of AI-III-52 bound inside the Top1 cleavage complex in the same orientation as in part B (left panel) or with a 90˚ rotation (center panel).The –1 base pair (capped sticks) covers and stacks against the entire drug (space filling).