Description_x000D_ Biochem/physiol Actions_x000D_ Human DNA topoisomerase I catalyzes the relaxation of both positive and negative supercoils without the requirement of energy. In addition to DNA replication and transcriptional activation, DNA topoisomerase I also plays a major role in pre-mRNA splicing, recombination, chromatin remodeling and other DNA or RNA templating activities. The C terminal domain of DNA topoisomerase I spanning from amino acids 713 to 765 is highly conserved and connected to the core domain by a poorly conserved linker domain (residues 636-713). An active site tyrosine has been characterized at position 723. Mutation of this tyrosine to phenylalanine at position 723 causes topo I to preferentially bind the supercoiled DNA rather than relaxed DNA in the mixture of supercoiled and relaxed DNA._x000D_ Physical form_x000D_ Clear and colorless frozen liquid solution_x000D_ Preparation Note_x000D_ Use a manual defrost freezer and avoid repeated freeze-thaw cycles. While working, please keep sample on ice.