Summary. Rat uteri from animals at pro-oestrus and Days 3–6 of pregnancy were incubated in vitro in the presence of [35S]methionine. Labelled endometrial proteins from each sample were separated by two-dimensional gel electrophoresis and resolved by fluorography. A group of 16 proteins that are synthesized only on specific days was defined. On Day 5, the day of embryo implantation, only 3 of these 16 proteins were synthesized. Of 8 proteins synthesized on both Days 3 and 6, 5 were also found on Day 4, but only 1 was synthesized on Day 5. These results demonstrate that during the interval in which implantation is initiated, no unique proteins are produced but several protein species are no longer synthesized by the endometrium.
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