Steroid hormones induce responses in target tissues by a mechanism involving the specific initial interaction of hormone with cytoplasmic receptor molecules. These receptors, usually localized in target tissues have high binding affinities and limited binding specificities for biologically active steroids. Examination of human leukemic lymphoblasts has revealed these receptors in some tumor samples. Their presence is well correlated with hormone responsiveness of the tumor in vitro. Similar studies on human breast cancer tumor homogenates has indicated that about 2 3 of primary tumors contain estrogen receptor. The absence of receptor predicts a lack of response to hormone therapy almost invariably, while the presence of receptor increases but does not assure that the tumor will be hormone responsive. Recently in vitro tissue culture systems which mimic the hormone responses observed in vivo have been developed which should significantly aid in the clarification of the mechanisms whereby steroid hormones stimulate and inhibit growth in target tissues.
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