The effects of tamoxifen, three of its in vivo metabolites and 3-hydroxytamoxifen on cellular proliferation and the induction of four oestrogen-regulated RNAs (pNR-1, pNR-2, pNR-25 and cathepsin D) have been measured in MCF-7 breast cancer cells in phenol red-free culture medium. This evidence concerns the gene CTSD and breast cancer.