Heat shock protein 90 (Hsp90) is a molecular chaperone involved in folding, assembly, maturation, and stabilization of the client proteins that regulate survival of malignant cells. As previous reports correlate high Hsp90 expression with decreased survival in breast cancer, Hsp90 may be a favorable target for investigational therapy in breast cancer. In our study, we have examined the response of a panel of both BRCA1-null (UACC 3199, HCC 1937, and MBA-MD-436) and BRCA1-wt breast cancer cell lines (MCF-7, MBA-MD-157, and Hs578T) to determine the proteins governing response to Hsp90 inhibitor 17-allyloamino-17-demethoxy-geldanamycin. On treatment with the drug, cells arrested at G(2)-M phase and entered aberrant mitosis in a BRCA1-dependent manner. Failure to arrest the cells at or before mitosis resulted in formation of micronucleated cells, aberrant segregation of chromosomes, microtubule misalignment, and multicentrosomes, leading in eventual mitotic catastrophe cell death. Our observations show that BRCA1 mediates G(2)-M transition mainly through chek1 on 17-allyloamino-17-demethoxy-geldanamycin treatment.