these receptor studies suggest that the effectiveness of this antiestrogen in antagonizing mammary tumor development and growth may reside in its ability markedly to perturb the distribution of er , maintaining over 90 % of er in the nucleus with concomitant low levels of cytoplasmic er , a situation that may render the mammary tissue insensitive to the animal 's own endogenous estrogens .
C0034804
er
C0007610
nucleus