Topisomerases are cellular enzymes that are vital for replication of the genome. However, if topisomerase and DNA form covalent complexes that prevent the resealing of DNA, this may lead to cell death. Essentially, this invention consists of a new isolated and cloned enzyme, tyrosyl-DNA phospodiesterase (TDP1) that is capable of hydrolyzing the covalent complexes between topisomerase and DNA, allowing the DNA to reseal. The mechanism that defines topiosomerases is their capacity to break DNA and, after an interval in which topological changes may occur, to reseal the break without the intervention of a high-energy cofactor. The breakage of the DNA is accompanied by the formation of a covalent bond between topisomerase and DNA to create an intermediate that is resolved during the resealing step. However, if the resealing step fails, the covalent intermediates between topoisomerase I and DNA can form complexes that lead to cell death. The failure of the resealing is increased by some chemotherapies such as camptothecin. Thus, this technology has many potential commercial uses including: a method for screening camptothecin analogues or other compounds for their resistance to repair by this enzyme or to prescreen patients for their sensitivity to topoisomerase inhibitors, which could identify patients most likely to respond to camptothecin therapy. Further, this invention provides for a vector comprising of the nucleic acid molecule for TDP1 as well as the method of altering the level of TDP1 in a cell, a tissue, an organ or an organism. Finally, this invention consists of a method for identifying a compound that stabilizes a covalent bond complex that forms between DNA and topoisomerase I, wherein the covalent bond cannot be cleaved.