In summary, with the possible exception of AU565 cells, (a) activity of -1797 p27 in either estrogen receptor (ER)-positive or -negative human breast cancer cells fairly faithfully recapitulated the breast cancer preventive activity in vivo of the various nutritional and chemopreventive anti-cancer agents and (b) the effective doses for the activation of -1797 p27 by these agents were within the range that had been found effective for in vivo rat models of breast cancer. This evidence concerns the gene ESR1 and cancer.