Our findings show that all 8 samples, derived from both the involved and the uninvolved breasts of a hereditary breast cancer patient develop normal epithelial architecture in vitro, implying that the epithelial/stromal (paracrine) interactions necessary for the development of this complex architecture are intact and normal in BRCA1 heterozygotes despite their greater risk of breast cancer. Here, BRCA1 is linked to breast carcinoma.