Abstract only e13677 Background: Triple negative breast cancer (TNBC) is strongly associated with germline BRCA1 pathogenic/likely pathogenic variants (PV/LPVs), with up to 28% TNBC patients being BRCA1 PV/LPV carriers. More recently, PALB2 variants have also been shown to be associated with TNBC. Genetic testing in Slovenian hereditary breast and ovarian cancer (HBOC) patients yields consistently high BRCA1/2 detection rates. Four BRCA1 PVs are particularly common, suggesting possible founder effects. Our aim was to analyze the results of panel based genetic testing in Slovenian TNBC patients, with an emphasis on BRCA1/BRCA2 mutational spectrum and detection rates depending on age at diagnosis. Methods: We evaluated the results of genetic testing, performed using a multi-gene panel in 1000 consecutive breast cancer cases. Only 210 patients with TNBC were included in further analysis. We analyzed the mutational spectrum and BRCA1/2 detection rates in TNBC patients stratified according to age at diagnosis. Results: In 80/210 (38.1%) TNBC patients PV/LPVs in BRCA1/2 were identified. Of these, the majority (74/80) were BRCA1-positive. The following variants: c.181T > G, c.5266dupC, c.1687C > T, and c.844_850dupTCATTAC were identified in 64.4% of BRCA1 carriers. BRCA1/2 detection rate decreased with age at diagnosis but was still 18.2% after age 60 (for BRCA1/2 detection rates stratified according to age at diagnosis see table). In 12/210 (5.7%) TNBC patients PV/LPVs in three other genes were identified (3.3% PALB2, 1.4% CHEK2, and 1% ATM). Conclusions: Prevalence of PVs in BRCA1 appears very high in Slovenian TNBC patients and warrants further population-based allele frequency studies with implications for future population screening. High rates of BRCA1/BRCA2 PVs in patients older than 60 years at diagnosis justify testing all TNBC patients, especially in view of treatment options which now include PARP inhibitors. PALB2 was the second most commonly mutated gene in our patients. Table: see text