A complementary DNA library was constructed from RNA of estrogenstimulated MCF-7 cells and screened for estrogen-regulated sequences. Four different messenger RNA sequences of varying abundance were isolated. Two of the sequences (pNR-3 and pNR-4) were induced approximately 2-fold, while the other two (pNR-1 and pNR-2) were induced at least 8-fold. The induction of both pNR-1 and pNR-2 requires similar physiological concentrations of estradiol and is near maximal at 10^-10 m. An increase in the levels of the RNAs is seen after 30 min of estrogen treatment, but pNR-1 reaches its maximal concentration faster than pNR-2. pNR-1 and pNR-2 were not expressed in all human breast cancer cell lines tested. pNR-1 was expressed and regulated by estrogen in the estrogen receptor-positive cell lines, MCF-7, T-47D, and ZR 75, whereas pNR-2 was not expressed in the T-47D cell line. pNR-1 and pNR-2 were not detected in two estrogen receptor-negative cell lines (BT20 and HBL 100). As the proliferation of the MCF-7, T47D, and ZR 75 cell lines is stimulated by estradiol, pNR-1 may provide a useful marker of hormoneresponsive breast cancer.