Antisense inhibition of BRCA1 expression in HBL100, MCF7 and T47D breast cancer cells led to increased sensitivity to mitomycin C and cisplatin, but not to doxorubicin and etoposide, as determined by a clonogenic assay. Similar results were obtained on the BRCA1-mutated HCC1937 breast cancer cell line. This effect was attributed to premature senescence of the chemosensitive cells. This evidence concerns the gene BRCA1 and breast cancer.