TOP1 and glioblastoma: The results from the other two sets of experiments with the GBM cell lines also confirmed that the TDP1/TOP1 activity ratio is a strong predictor for irinotecan IC50 (Figure S5, Table S2), with Pearson correlation coefficient R = 0.972 for the average of the gel-based TDP1/TOP1 activity ratios from all three sets of experiments.