P27(KIP1) expression indicates that steroid receptor-positive cells are a non-proliferating, differentiated subpopulation of the normal human breast epithelium.

To test the hypothesis that steroid receptor-expressing cells are derived from the proliferative population, we examined expression of the p27(KIP1) inhibitor of cyclin-dependent kinase activity (a differentiation marker) while tracking the fate of proliferating cells in normal human breast tissue implanted into athymic nude mice using tritiated thymidine [3H]-dT. We identified a small number of cells that appeared to have divided just once before switching on p27(KIP1) expression. p27(KIP1)+ve cells also expressed steroid receptors, but not the Ki67 proliferation-associated antigen. These data support the hypothesis that steroid receptor-expressing cells are a differentiated population within the normal human breast epithelium.