The mts1 (S100A4) gene, encoding a Ca(2+)-binding protein of the S-100 subfamily, is involved in the control of tumor metastasis in some murine tumor cell lines. To further analyze its role, we transfected hormone-responsive human breast cancer MCF-7 cells with the mts1 gene under the control of a strong constitutive promoter. All of the 3 tested clones (MCF-7/mts1) producing Mts1 protein acquired an ability for hormone-independent growth in nude mice. Tumors derived from mts1 transfectants revealed local invasiveness into surrounding muscle and adipose tissues and metastasized to regional lymph nodes and lungs, characteristics which are rarely observed with parental MCF-7 cells. Electron-microscopic analysis of MCF-7/mts1 cells demonstrated structural changes in anchoring junctions, particularly in intermediate filament attachment site (desmosomes). The mts1-transfected clones expressed estrogen receptor, and their growth in tissue culture was both estrogen- and anti-estrogen responsive. Changes in regulation of the estrogen-dependent proteins progesterone receptor and cathepsin D were observed in some of the transfected clones. Our results indicate that mts1 expression in human breast cancer cells induces several changes characteristic of malignant phenotype and tumor progression.