BRCA1 is inactivated by gene mutations in >50% of familial breast and ovarian cancers. BRCA1 is primarily a nuclear protein, although others previously reported cytoplasmic staining in breast tumor cells. In this study, we demonstrate the cytoplasmic mislocalization of BRCA1 caused by a subgroup of clinically relevant cancer mutations. We show that mutations that disrupt or delete the C-terminal BRCT domains, but not other regions of BRCA1, caused significant relocalization of BRCA1 from nucleus to cytoplasm. Two of the BRCT mutations tested (M1775R and Y1853X) are known to adversely affect BRCA1 protein folding and nuclear function. The BRCT mutations reduced BRCA1 nuclear import by a mechanism consistent with altered protein folding, as indicated by the restoration of nuclear staining by more extensive C-terminal deletions. Furthermore, we observed increased cytoplasmic staining of both the ectopic and endogenous forms of the BRCA1-5382insC mutant (deleted BRCT domain) in HCC1937 breast cancer cells. Unlike wild-type BRCA1, the BRCA1-5382insC mutant failed to form DNA damage-inducible foci when targeted to the nucleus by BARD1. We propose that BRCT mutations alter nuclear targeting of BRCA1, and that this may contribute to the inhibition of nuclear DNA repair and transcription function.