We have isolated a number of subclones from the human MCF-7 breast cancer cell line, with different metastatic potential together with various degrees of endocrine resistance. The subclones were selected for progression in vivo in nude mice, as well as in vitro by selecting MCF-7 cells for growth in the absence of estrogen (E₂). The phenotype of the subclones range from the non-metastatic hormone dependent cell line to a multi hormone resistant and metastatic cell line (phenotypes of cell lines shown in table). E₂ Melastatic 4-OHTAM* ICI 182.780* MCF-7 Dependent - Sensitive Sensitive MCF-7/LCC1 Independent + Sensitive Sensitive MCF-7/LCC2 Independent -M- Resistant Sensitive MCF-7/LCC9 Independent + Resistant Resistant * Anti-estrogen In order to investigate genes involved in metastasizing and endocrine resistance the technique of Differential Display (comparison of cDNA, amplified by PCR, from several cell lines) was used. By this technique it was possible to detect a number of differentially expressed mRNAs. We isolated a cDNA (MCF7/LCC9p2) that was expressed in the metastasizing MCF-7/LCC9 cell line and not in the non-invasive MCF-7 cell line. This finding has been verified both on mRNA and Protein level by RT-PCR and Western blotting. By using the sequence information from the cDNA , it was possibly to clone a full length human sequence of the gene by RACE techniques. Search in Genebank revealed a homolog gene cloned from a regenerating rat liver. Analysis of the amino acid composition reveals several Hydrophobie domains which indicate that the protein could be membrane bound. This theory is also supported by the fact, that is was only possible to detect the protein by western analysis in the detergent phase. The function of the gene is so far unknown but it could be involved in the mechanism behind invasion or in the resistance against anti-estrogens.