Immunohistochemical double staining with estrogen receptor (ER) and epidermal growth factor receptor 2 (HER2) was conducted in tissue sample of 125 women with invasive breast cancer. The age at the time of surgery ranged from 28 to 82 years. The tumor size was 2 cm or less in 42 patients and larger than 2 cm in 83. Axillary lymph node status was positive in 53 patients and negative in 72. Estrogen receptor (ER) which was measured using enzyme immunoassay (EIA) was positive in 67 patients, negative in 50 and unknown in 8. Of the 125 patients evaluated, 83 (66.4%) were immunohistochemically positive for ER. ER by immunohistochemistry (IHC) (ER-IHC) was significantly (p<0.01) correlated with ER by EIA (ER-EIA). ER-EIA values in ER-IHC scores were 1.4 fmol/mg protein in score 0, 0.0 in score 1, 19.0 in score 3, 21.2 in score 4, 12.2 in score 5, 17.6 in score 6, 30.0 in score 7 and 114.8 in score 8. ER-EIA values in ER-IHC-score 8 were significantly higher than in scores 0, 2, 3, 4, 5, 6 and 7. Of the 125 patients, 35 (28%) were immunohistologically positive for HER2. HER2 expression was inversely correlated with ER expression. When evaluated even in ER-positive patients, HER2 overexpression was associated with lower ER levels. In this study, we conducted immunohistochemical double staining with ER and HER2, and demonstrated that low ER levels might be one factor in the relative resistance of HER2-positive and ER-positive tumors to hormonal therapy.

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