Fig. 2. Signaling mechanisms studied in monotypic 3D cultures of human mammary epithelial cell lines. The phenotypes of human mammary epithelial cells can be readily distinguished in the context of 3D lrBM assays. After 10 days in culture, non-malignant cells form growth-arrested, polarized acini with central lumens, whereas malignant cells form apolar colonies of continually growing cells that vary in size and shape depending on the degree of tumorigenicity. A number of studies, some of which are depicted here, have utilized this assay to explore the molecular regulation of normal breast (e.g., lumen formation) as well as aberrant signaling during tumor progression and/or reversion. Individual studies are referenced in the text.