[A,B] Cell death was assayed by ELISA in tumor cells at 2 d after treatment with TOP2 poisons. Shown are average results of 3 independent experiments. [A] Vector-control, +DLX4 and +TOP2α MDA-MD-468 cells were incubated without and with doxorubicin (100 nM). [B] TOV112D cells were transfected with empty vector, non-targeting shRNA, shRNAs targeting DLX4 (shA-DLX4, shB-DLX4) and TOP2A (shA-TOP2A, shB-TOP2A). At 1 d thereafter, doxorubicin (200 nM) or etoposide (500 nM) was added to cells. [C,D] DSBs were assayed by neutral comet assay in [C] MDA-MB-468 and [D] TOV112D cells at 1 d after doxorubicin treatment. Shown are mean tail moments of 3 independent assays where 50 randomly selected cells were scored in each assay.