We show that a mouse uterus nuclear phosphatase exists that is capable of inactivating nuclear oestrogen receptor complexed to oestradiol-17 beta in vitro but is ineffective when the receptor is complexed with the two non-steroidal anti-oestrogens, nafoxidine and tamoxifen. We suggest that the long half-life of the tamoxifen-receptor complex versus the short half-life of the oestradiol-receptor complex in uterine nuclei in vivo is the result of the ineffectiveness of the phosphatase in dephosphorylating the anti-oestrogen-receptor complex.

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