TOP1 and cancer: The anti-cancer drug camptothecin specifically inhibits Top1 [3] by acting prior to the resealing step, effectively trapping Top1 covalently bound to the DNA in a “cleavable complex.” Camptothecin and other Top1 poisons are used for the treatment of ovarian, cervical, colon, pancreatic, lung, breast, prostate and brain cancers [4].