To investigate radiation effects on BRCA1-deficient cells, we knocked down BRCA1 expression in MCF7 breast cancer cells using small interfering RNA (siRNA) and assessed survival of the resulting BRCA1-knockdown cells using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assays following exposure to increasing doses of irradiation (Fig. 1A). Here, BRCA1 is linked to breast carcinoma.