High-quality WGS (34.5X average sequencing depth, ≥ 20X coverage across 94.9% of the genome) on tumors and a blood sample revealed missense mutations in human breast cancer-related genes such as BRCA2 and TP53. Furthermore, we found evidence of canine-specific somatic mutations depending on the tumor subtypes, including HECTD4 in malignant tumors and NIPBL in epithelial-derived malignant tumors. Here, NIPBL is linked to breast cancer.