The mechanism of BRCA1 tumor suppression in human breast and ovarian cells is the focus of intense investigation. In this report, full length BRCA1 (230 kDa) introduced into cells with CMV promoter constructs was nuclear when transgene expression was low whereas high expression resulted in cytoplasmic accumulation, aberrant nuclear and cell morphology. A nuclear localization signal (NLS) was mapped to BRCA1 amino acid positions 262-570. We describe a splice variant, BRCA1-delta11b, missing the majority of exon 11 including the NLS. Exogenous BRCA1-delta11b (110 kDa) was cytoplasmic and, unlike the full-length protein, overexpression of the protein encoded by the variant did not appear to be toxic. RNA probe titrations and RT-PCR demonstrated that BRCA1 and delta11b transcripts are coexpressed in a wide variety of cells and tissues. Interestingly, BRCA1-delta11b message was greatly reduced or absent in several breast and ovarian tumor lines relative to exon 11 transcripts and a delta9,10 splice variant. Taken together our results suggest that full-length BRCA1 and BRCA1-delta11b may have distinct roles in cell growth regulation and tumorigenesis.