BRCA1 plays a role in DNA repair and transcription, although the importance of the transcriptional control to the clinical phenotype is unclear. To identify transcriptional targets of BRCA1, we assessed differential gene expression in mouse embryonic stem cells with deletion of both BRCA1 alleles using high-density microarrays. Several groups of genes were downregulated in the knockout cell line, including some stress response genes, cytoskeleton genes and genes involved in protein synthesis and degradation. Of particular interest was the finding that the 14-3-3 σ checkpoint control gene was significantly downregulated in BRCA1−/− cells when compared with wild-type embryonic stem cells or BRCA1−/− embryonic stem cells reconstituted with a BRCA1 transgene. Irradiation of BRCA1−/− cells showed much lower levels of 14-3-3 σ induction. Moreover, when these cells were treated with ionizing radiation their cell cycle profile showed an inability to sustain the G2/M growth arrest typical of cells deprived of 14-3-3 σ. We found that BRCA1 synergistically activates p53-dependent transcription of the 14-3-3 σ gene and that this activation is dependent on an intact p53, BRCA1 and p53 response elements. These data indicate that BRCA1 is an important transcriptional cofactor in p53-mediated responses to DNA damage and may explain some of the clinical and laboratory presentations of cancers in BRCA1 carriers.