This study has shown that breast tumourigenesis in women with pathogenic variants in BRCA2, CHEK2, PALB2, BRCA1 and TP53 can involve a DCIS precursor stage and that the focus of genetic testing in DCIS should be on women under the age of 40 with ER-positive DCIS. Here, BRCA1 is linked to ductal breast carcinoma in situ.