BRCA1 acts as a tumour suppressor and germ-line mutations within this gene are found in a large proportion of families with breast cancer. The aim of our study was to unravel the mechanism of action of genistein, the major soy phytoestrogen, in BRCA1-mutant human breast cancer cell lines. Four breast cancer cell lines were studied for their response to genistein, three of them harbouring different mutations within the BRCA1 gene (HCC1937, SUM149 and SUM1315 cells) and the MDA-MB-231 cell line, which expresses a functional BRCA1 protein. We showed that genistein inhibits proliferation and induces apoptosis more efficiently in BRCA1-mutant cells than in cells expressing wild-type BRCA1 protein. Increased AKT and decreased p21(WAF1/CIP1) protein levels could explain the relative resistance to genistein elicited by cells with wild-type BRCA1. BRCA1-mutant breast cancer cells are highly sensitive to genistein treatment and p21(WAF1/CIP1) and AKT could be genistein targets in these cells.