INTRODUCTION AND OBJECTIVES: BRCA1 is a tumor suppressor gene that plays a central role in DNA repair by homologous recombination. Carriers of pathogenic mutations in BRCA1 are predisposed to the hereditary breast and ovary cancer syndrome (HBOC). BRCA1 encloses two tandem BRCT domains (tBRCT) in its C-terminal region. The tBRCT domain is crucial for BRCA1-mediated tumor suppression as the loss of its last 11 amino acid residues results in cancer predisposition. Thus, nonsense or frameshift variants are usually classified as pathogenic. However, missense mutations, small in-frame deletions and/or insertions (indels) are known as variants of uncertain significance (VUS). Functional assays are powerful tools for assessing the impact of variants on specific protein function. Our group showed that BRCA1 tBRCT domain integrity correlates to its transcriptional activity (TA). We have previously evaluated more than 300 missense variants enclosed within the C-terminal region (1396-1863 amino acid residues, coded by exons 13 to 24) of BRCA1 using the TA assay. However, little is known about the functional impact of small in-frame indels and nonsense mutations on the C-terminal border of the tBRCT domain. MATERIAL AND METHODS: In-frame indels previously reported in the population were identified through the BRCA Exchange platform. We selected and generated all 16 in-frame indels reported in the 13/24 region that resulted in single amino acid variation (SAV) or single codon deletion (SCD). We also generated 8 nonsense truncation variants in the C-terminal border of the tBRCT domain. Variants were generated by site-directed mutagenesis using pCDNA3:GAL4 DNA binding domain (DBD):BRCA1 as template. The functional impact of variants was interrogated by the TA assay in human HEK293FT cells using known benign (S1613G) and pathogenic (M1775R and Y1853X) variants as controls. Protein expression was evaluated by immunoblotting. RESULTS AND CONCLUSION: All variants were successfully generated and evaluated by TA assay and immunoblotting. Indels located within the tBRCT domain (amino acid 1650 to 1863) that resulted in SAV presented up to 50% of WT activity. However, SAV in the disordered region next to the tBRCT presented activity that exceeded 100% of wild-type (wt), similar or higher than benign variant S1613G. Similarly, the SCD in the tBRCT domain resulted in complete loss of TA. The SCD located in the disordered region presented activity ranging from 55% to 110% of wt activity. Interestingly, the loss of a single amino acid in the C-terminal border of the tBRCT resulted in a 50% decrease in TA activity. This phenotype is also observed in previous studies interrogating the absence of 2 to 6 amino acids. However, the loss of residues P1856 and Q1857 showed markedly reduction in TA. Collectively, our data corroborate the notion that TA correlates to tBRCT domain integrity. We also show that the disordered region seems to tolerate SCD without affecting TA.