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BACKGROUND: Breast cancer is mainly divided into estrogen receptor (ER)-positive luminal and ER-negative basal-like tumors. Luminal-type tumors are associated with better survival and respond to hormone therapies whereas basal-like tumors are more aggressive and associated with a poor prognosis. Mammary epithelia are mainly composed of luminal and basal cells that are maintained by luminal and basal progenitors, respectively. The maintenance of luminal cell fate is orchestrated by networks of transcription factors, including BRCA1 and GATA3. Functional loss of BRCA1 by germline or somatic mutation or by promoter methylation is associated with more than one third of basal-like breast cancers. GATA3 expression is reduced in basal-like breast cancers and cancers that metastasize. Overexpression of GATA3 in cancer cells inhibits tumor formation. Deletion of Brca1 or Gata3 in mice results in early lethality or growth defects. How BRCA1 and GATA3 suppress breast cancer remains elusive. METHODS: We generated mice lacking Brca1 or Gata3 in mammary epithelia. Due to the proliferative defects and induction of p18Ink4c (p18), an inhibitor of CDK4/6, in mammary epithelial cells of these mice, we then generated mice lacking Brca1 or Gata3 in p18 deficient mammary epithelia. We determined spontaneous mammary tumor development in mutant mice and the mechanisms underlying the role of Brca1 and Gata3 in suppressing tumorigenesis and progression. RESULTS: Depletion of Brca1 or Gata3 led to growth defects of mammary epithelial cells, which was rescued by loss of p18. Depletion of Brca1 or Gata3 in a p18 null background induced heterogeneous mammary tumors with less luminal and more basal-like features and accelerated metastasis. Deletion of Brca1 eliminated Gata3 expression in human and mouse mammary tissues and cells. How Brca1 interacts with Gata3 to control mammary tumor development and progression is currently under investigation. CONCLUSION: Our results suggest that loss of function of either Brca1 or Gata3 induces basal-like mammary tumors in p18 deficient background. Citation Format: Pei X-H, Chan HL, Zhang L, Wang C, Robbins DJ, Capobianco T, Bai F. Loss of function of Brca1 or Gata3 induces basal-like breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-08-01.