Studies on the endogenous activation and inhibition of hormone receptor reaction.

Activation and inhibition of estrogen and progestin receptor functions by endogenous biological factors present in the cytosol preparations were demonstrated by assaying mixed cytosol preparations from human breast tumors and immature rabbit uteri. Both activation and inhibition of receptor-hormone binding reaction could be observed by tumor-rabbit uterus or tumor-tumor combinations. The activation of tumor progestin receptor by rabbit uterus cytosol was markedly profound.