Schematic representation of the repression/amplification model in mammary epithelial cells. In these cells ERBB2 expression is regulated. A crosstalk between ER and ERBB2 pathways is an important feature of these cells. ERBB2 is normally expressed at low level in ER-negative cells (A). In ER-positive cells, ERBB2 expression is quenched by ER-induced transcriptional repressors (B). In ERBB2-positive tumors, amplification titrates out the repressors and allows overexpression of ERBB2; this may in turn shut down ER expression (C).