BRCA1 and ductal breast carcinoma in situ: Information derived from studies of human cancers has been largely limited to retrospective analyses of prevalent changes in established tumors (Futreal et al,2004; Nik‐Zainal et al,2012), or a few analyses of preneoplastic mammary cells were obtained from carriers of BRCA1 mutations (Lim et al,2009; Proia et al,2011; Choudhury et al,2013) or from samples of ductal carcinoma in situ (DCIS; Yeong et al,2017).