We have evaluated the transcriptional activation of a human p21 promoter reporter construct by transfection of BRCA1 expression constructs into tumorigenic and nontumorigenic human breast cell lines. Two cell lines with wildtype p53 (MCF-7 and MCF10A) demonstrated transcriptional activation of the p21 promoter by full-length BRCA1 (BRCA1L) as well as by two splice variants that lack most of exon 11 (BRCA1S and BRCA1S-9,10). In contrast, two cell lines with mutant p53 (MDA-231 and HCC1937) were inactive. Co-transfection of BRCA1L with BRCA1S or BRCA1S-9,10 exhibited synergistic p21 promoter activation, due to augmented expression of the cytomegalovirus promoter-based BRCA1 expression constructs. We examined the transcriptional activity of two known sequence alterations in BRCA1, one that results in a carboxy-terminal truncation of BRCA1 and is clearly pathogenic, and the other a missense mutation that is suspected of predisposing to cancer. Although both mutations have been shown to be defective in some assays of transactivation, we observed both mutations to be fully active in activation of the p21 promoter when incorporated in the full-length BRCA1L. In contrast, these mutations rendered BRCA1S inactive. These observations indicate that such transcriptional assays cannot serve as the basis for a functional appraisal of BRCA1 sequence alterations encountered in the course of genetic testing.