Cells from normal human breast epithelium were cloned in monolayer culture and the clones were stained with monoclonal antibodies. Tissue was from reduction mammoplasty operations. Cloning efficiencies were 5–30%. Two types of clone were identified: 10 to 30% were of relatively spread cells whose boundaries were often difficult to see by phase-contrast microscopy but where they were visible they appeared as dark lines. The edges of the clones usually appeared to be under tension. These clones were stained by two monoclonal antibodies, LICR-LON-M8 and M24, that stain luminal epithelial cells in the intact tissue, but not myoepithelial or stromal cells. Within a clone the cells showed a full range of antigenic phenotypes. This was confirmed for clones grown from single cells that had been isolated manually. The second type of clone was more compact with little evidence of tension at the edges, and cell boundaries were clearly visible and bright under phase contrast. These clones were not stained by antibodies M8 or M24. Both types of clone stained with a third monoclonal antibody that is specific for luminal epithelial cells in the intact tissue, LICR-LON-M18, but the distribution of staining was different in the different types of clone. The simplest interpretation of the two types of clone is that luminal epithelial cells give rise to the spread type of clone while the myoepithelial cells give rise to the more abundant and vigorous compact clones. Alternatively, the compact clones may be from luminal epithelial cells that have lost differentiated characteristics.

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