The mechanism of cell immortalization of human breast epithelial cells leading to neoplastic transformation is not clear. The isolation and characterization of a spontaneously immortalized human breast epithelial cell line, MCF-10F, have provided a valuable tool to identify genes involved in this process. Using the technique of differential display, we have identified seven cDNA bands differentially displayed in the MCF-10F cells when compared with the mortal S130 cells from which MCF-10F was originated. One of these bands was isolated and cloned. Sequence analysis revealed 99% homology to the EF-hand calcium-binding protein S100P (Placental). The clone was overexpressed in the immortal cell line MCF-10F when compared to the mortal counterpart S130 or other primary cultures of human breast epithelial cells. In addition, it was highly expressed in chemically transformed breast epithelial cell lines (BP1E and D3. 1), breast cancer cell line T47D, as well as in three invasive ductal carcinomas when compared to their normal adjacent tissue. The S100P protein was localized by immunohistochemistry, using a monoclonal antibody against the same amino acid sequence of the gene cloned, in ductal hyperplasias, in situ and invasive ductal carcinoma, but not in the normal tissues. We concluded that S100P overexpression is an early event that might play an important role in the immortalization of human breast epithelial cells in vitro and tumor progression in vivo.