Previous studies suggest that topoisomerase I (Topo I) plays a critical role in cell growth. However, the structure of the Topo I gene has not yet been determined. Two complementary DNA (cDNA) clones for the human Topo I 4.1-kilobase mRNA were isolated independently from HeLa and KB cell cDNA libraries. These clones were identical and they contained 679 base pairs of coding and 1138 base pairs ...