In the laboratory, tamoxifen-stimulated growth of breast cancer cells
occurred within a year of tamoxifen treatment. These tumors grew in response to
tamoxifen, although they continued to express estrogen receptor. In some cases,
however, these tumors became supersensitive to very low levels of estrogen,
which then produced a profound antitumor effect. Further study of these
biological processes in the laboratory may, later, prove useful in the clinical
setting.
