In order to examine whether the expression of calcium-binding proteins of the SI00 family may correlate with the transformation grade of human mammary-tumor cells, we studied the expression patterns of 4 members of this family (CACY, CAPL, S100L, S100α/β) in human breast-cancer cell lines. Each S100 protein is shown to be individually regulated in the human breast-cancer cell lines we studied, but it appears that the expression levels of S100 proteins do not strictly correlate with prognostic factors or the tumorigenicity of the cells. However, 2 aggressive cell lines, MDA-MB-231 and HS-578T, show elevated expression of CAPL, We show that methylation may account for partial regulation of the SI00 genes, whereas neither genomic rearrangements in the S100 gene cluster region nor gene dosis effects seem to influence their expression pattern in MDA-MB-231 and MCF-7 cells. On the basis of our genomic analyses, we can localize the gene for SIOOL within 5 kb upstream of S100E, thus extending the SIOO gene cluster by one gene. A series of primary breast tumors was collected and tested for expression of CAPL, CACY and S100α/β. The results show that all human breast-cancer tissues tested express CACY, whereas the presence of CAPL is more restricted. There if a significant correlation between enhanced expression of CAPL and presence of the invasivity marker urokinase-type plasminogen activator (uPA). This observation suggests that CAPL may play an important role in the acquisition of metastatic potential of human mammary epithelial cells.