Although similar treatment patterns were seen within biologic subtypes of T1N0 disease regardless of germline pathogenic variant (85–90% of T1N0 TNBC patients received chemotherapy compared to 52–60% of ER + HER2- breast cancers), the higher prevalence of TNBC in BRCA1 carriers resulted in 80% of BRCA1 patients receiving chemotherapy for stage I breast cancer overall. This evidence concerns the gene ERBB2 and breast cancer.