Among all 411 patients, 135 (32.8%) carried a pathogenic or likely pathogenic heterozygous germline mutation in 19 genes, including high-penetrant breast cancer genes like BRCA1 (43–31.9%), BRCA2 (19–14.1%), PALB2 (12–8.9%), TP53 (3–2.2%), and CDH1 (2–1.5%), and moderate to low-penetrant genes CHEK2 (25–18.5%), ATM (12–8.9%), MMR group PMS2 (2–1.5%), MSH3 (1–0.7%), MSH6 (1–0.7%), MLH1 (1–0.7%), BARD1 (3–2.2%), NF1 (3–2.2%), MUTYH (5–3.7%), EGFR (1–0.7%), SDHB (1–0.7%), RAD50 (3–2.2%), NBN (2–1.5%), and XRCC2 (2–1.5%). This evidence concerns the gene MLH1 and breast cancer.