In vitro TDP1 catalytic activity correlates positively with TDP1 protein levels in colorectal cancer cell lines. In vitro TDP1 catalytic activity correlates positively with TDP1 protein levels in colorectal cancer cell lines. A, diagram depicting TDP1 processing of a 5′-Cy5.5-labeled oligonucleotide substrate harboring a 3′-phosphotyrosine modification (3′-PY) to form a Cy5.5-labeled oligonucleotide product harboring a 3′-phosphate (3′-P) that is smaller in size. B, 10-μL reaction volumes containing Cy5.5-labeled substrate and the indicated amounts of recombinant human TDP1 (2 pmol/L) or colorectal cancer WCE (ng) were separated on a 20% Urea SequaGel and imaged at 635 nm. The substrate (3′-PY) and product (3′-P) are indicated by arrows. C, reaction products were quantified using the ImageJ software and the average percentage of cleavage for three independent experiments ± STD is shown. A graphical representation of the Pearson correlation coefficient (R) between colorectal cancer TDP1 protein levels and percentage of cleavage for 30 ng WCE (D) or 60 ng WCE (E). Cornelia Meisenberg et al. Mol Cancer Ther 2015;14:575-585 ©2015 by American Association for Cancer Research