The binding of estradiol by breast carcinoma cytosol was studied in about 100 patients. Excess free estradiol was separated from bound estradiol by charcoal adsorption. Three parameters were used to describe the binding: the ratio of bound tritiated estradiol to total tritiated estradiol in the incubation medium; the inhibition of the binding of tritiated estradiol by the addition of nonradioactive estradiol to the incubation medium; the slope of the steepest part of the curve in a Scatchard plot of [(bound estradiol/mg protein)/estradiol in incubation medium]/[bound estradiol/mg protein]. It is suggested that the last method, which permits the estimation of the association constant of the estradiol-receptor complex and is independent of partial saturation of the receptor sites by endogenous estradiol, probably provides the best distinction between hormone-dependent and hormone-independent breast carcinomas. Comparison of the binding of estradiol by breast carcinoma cytosol with that of surrounding normal breast tissue showed that even high-binding, “hormone-dependent” carcinoma is surrounded by “hormone-independent” normal breast tissue.

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This work was supported by grants from the A. Yeldham and M. Raine Medical Research Foundation, the Cancer Council of Western Australia, and the King Edward Memorial Hospital for Women.

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