Estrogen receptor (ER), progesterone receptor (PR) and creatine kinase (CK) were measured in cancerous tissue from 29 post-menopausal patients with endometrial carcinoma under basal conditions and after a short course of tamoxifen treatment. ER and PR were detected in nearly all tumors. CK was detected in all of the tumors examined. After tamoxifen, PR and CK increased simultaneously in 26% of cases, while they were either enhanced, decreased or unmodified in the remainder. No correlation could be found between increase of PR and tumor differentiation. CK, however, was enhanced only in the more differentiated cancers. These results indicate that only a percentage of endometrial cancers are responsive to tamoxifen. It is hypothesized that patients bearing these tumors are those likely to benefit from endocrine therapy.
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