BRCA1 and metastatic neoplasm: To further identify the driver mutations that might be missed by bulk sequencing, understand how tumors evolve from premalignant mammary glands upon loss of Brca1 and find early lesions that may serve as driver mutations for tumor initiation, progression, and metastasis, we decided to conduct scWES on samples from different stages prior to tumor formation (4-month-old virgin and tumor-adjacent mammary tissues), as well as on PT and metastatic tumor samples (Fig. 3a; Supplementary Data 1).