Figure 1

Examples illustrating typical levels of estrogen receptor alpha (ER) expression in normal and premalignant breast epithelial cells (dark nuclei, ER-positive as assessed by immunohistochemistry). On average, (a) normal premenopausal terminal duct lobular units contain about 30% ER-positive cells. In contrast, nearly all cells express very high levels of ER in the majority of premalignant breast lesions, including (b) atypical ductal hyperplasia, (e) atypical lobular hyperplasia, (f) and lobular carcinoma in situ. Ductal carcinoma in situ is an exception, showing a continuum of ER expression ranging from (c) very high levels characteristic of histologically lower-grade lesions to very low levels characteristic of higher-grade lesions, including a substantial subset (~25%) that is entirely ER-negative (d).