DNA topoisomerase II beta cDNAs from Chinese hamster lung cells sensitive (DC-3F) and resistant to 9-OH-ellipticine (DC-3F/9O-HE) were isolated. In the sensitive cells, the sequence defines an open reading frame of 4839 nucleotides, and extends over a 323 nucleotides untranslated region up to the putative polyadenylation site. The deduced amino acid sequence predicts a protein with 1612 amino acids in length and a calculated molecular mass of approx. 182 kDa. The cDNAs from the resistant cells only differs by one mutation at position length and a calculated molecular mass of approx. 182 kDa. The cDNAs from the resistant cells only differs by one mutation at position 1710 which converts a Trp codon (TGG) to a stop codon (TGA). This mutation accounts for the loss of DNA topoisomerase II beta in the 9-OH-ellipticine resistant cells.