A hydroxylapatite micromethod for measuring estrogen receptor in human breast cancer.

The concentration of estrogen receptor (ER) in a human breast tumor is a critical variable predicting the response to endocrine therapy and the course of the disease. Since many tumor specimens are quite small, a reliable and simple ER assay requiring a minimum of tissue is desirable. We here describe a hydroxylapatite assay for ER that (a) requires only a single, saturating concentration of [3H]estradiol, (b) agrees with more complex multiple-concentration assays and with the standard dextran-coated charcoal assay at normal protein concentrations, (c) is far more reliable than the latter at low protein concentrations, and (d) can be adapted to an accurate and reliable ER microassay requiring less than 50 mg of tissue.