Breast cancer begins in which cell layer?

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Breast cancers most often arise from the epithelial lining of the ducts and lobules—the inner cell layer that forms the ductal and lobular structures of the breast. These epithelial cells can accumulate genetic changes that lead to malignant transformation, producing ductal or lobular carcinomas. The basement membrane sits beneath this epithelium and serves as a barrier; cancer typically starts within the epithelial layer and may invade through the basement membrane into surrounding stromal tissue as it progresses. Myoepithelial cells lie outside the epithelial layer and mainly provide structural/contractile support, but they are not the origin of most breast cancers. The stromal tissue is connective tissue around the ducts and lobules; cancers arising there are far less common.