The resulting tumors were detected with a latency of 4.9 months6, similar to 4.3 months in rats injected by AAV-edited Pik3ca (Fig S5), but these mouse tumors are all characterized as low to moderate grade ductal carcinomas with focal squamous differentiation and negligible proportions of ER+ or PR+ cells (Fig 4B), dramatically different from those of rat lesions induced by genome editing of Pik3ca, which are benign fibroadenoma featuring normal-appearing ducts and without any metaplastic changes. Here, ESR1 is linked to breast ductal adenocarcinoma.