D MCF-7 cells were transfected with the indicated siRNAs. Cell sensitivity to camptothecin (CPT), hydroxyurea (HU), etoposide (ETO), or ionizing radiation was determined by MTS assays. Data are presented as mean ± SD of three biological replicates. Two-tailed Student's t test, *P < 0.05, **P < 0.01, NS: no significant difference. 