Embryonic Barcoding of Equipotent Mammary Progenitors Functionally Identifies Breast Cancer Drivers

Identification of clinically relevant drivers of breast cancers in intact mammary epithelium is critical for understanding tumorigenesis yet has proven chal- lenging. Here, we show that intra-amniotic lentiviral injection can efficiently transduce progenitor cells of the adult mammary gland and use that as a plat- form to functionally screen over 500 genetic lesions for functional roles in tumor formation. Targeted pro- genitors establish long-term clones of both luminal and myoepithelial lineages in adult animals, and via lineage tracing with stable barcodes, we found that each mouse mammary gland is generated from a defined number of ?120 early progenitor cells that expand uniformly with equal growth potential. We then designed an in vivo screen to test genetic inter- actions in breast cancer and identified candidates that drove not only tumor formation but also molecu- lar subtypes. Thus, this methodology enables rapid and high-throughput cancer driver discovery in mammary epithelium.