To follow up on our observed correlation between irinotecan sensitivity and the TDP1/TOP1 activity ratio in GBM cell lines, we elevated the TDP1 activity in H4 cell line by transfection with clones expressing wild-type (WT)-TDP1 or null activity mutant H263A-TDP1 [19] to determine the effect on irinotecan IC50. Here, TDP1 is linked to glioblastoma.