INTRODUCTION AND OBJECTIVES: BRCA1 (Breast Cancer 1, early onset) was the first gene linked to breast and ovarian cancer predisposition. Still, the risks conferred by a significant portion of BRCA1 variants identified in the population remain unknown. Most of these variants of uncertain significance (VUS) are missense alterations. However, the functional implications of small in-frame deletions and/or insertions (indels) are also difficult to predict. BRCA1 encloses two tandem BRCT domains (tBRCT) in its C-terminal region. The tBRCT domain is crucial for the BRCA1-mediated tumor suppression as the loss of its last 11 amino acid residues results in cancer predisposition. Our group has previously evaluated the functional impact of 347 missense variants using a validated transcriptional activity (TA) assay based on a fusion of the GAL4 DNA-binding domain (DBD) to the BRCA1 C-terminus. Here we tested the functional impact of 30 naturally occurring small in-frame indels located at the C-terminal region of BRCA1. MATERIAL AND METHODS: All variants previously reported in the population were identified through the BRCA Exchange platform. We considered small in-frame indels all genomic variants that resulted in substitution, deletion and/or insertion of up to four amino acid residues. We selected all 30 small in-frame indels located within exons 13 to 24 for further functional evaluation by TA assay. Variants were divided into three groups based on the protein outcome: (1) indels that resulted in missense variations; (2) small deletions (SD); and (3) variants that resulted in small insertions (SI). In addition, we designed four single codon deletions at the tBRCT linker and six nonsense variants at the C-terminal border of the tBRCT domain to better characterize these regions. Variants were generated by site-directed mutagenesis using the pCDNA3_GAL4DBD_BRCA1 as a template. The functional impact of all variants was interrogated by the TA assay in human HEK293FT cells using known benign (p.S1613G) and pathogenic (p.M1775R and p.Y1853X) variants as controls. Protein expression profiles were evaluated by immunoblotting. RESULTS AND CONCLUSION: Our data corroborate previous observations that missense variants situated out of the tBRCT present a modest impact on BRCA1 TA function. We show that SD and/or SI located at the linker region and tBRCT C-terminal border present a less prominent impact on the BRCA1 TA function. On the other hand, indels within one of the two BRCT have a critical impact on BRCA1 TA activity. Further, by testing additional single codon deletions, we show that the linker region can tolerate the absence of specific residues. However, the C-terminal border of the tBRCT is sensitive even to the loss of its last amino acid residue (Y1863). Our data highlights the importance of assessing the impact of small in-frame indels in BRCA1 to improve risk assessment and clinical decisions for carriers.