To investigate potential molecular changes in breast cancer cells, induced by co-existing adipocytes, we established a two-dimensional co-culture system of three different human breast cancer cell lines (T47D; MCF-7: ER+/PR+/HER2−; MDA-MB-231: ER−/PR−/HER2−) and murine 3T3-L1 adipocytes. This evidence concerns the gene ERBB2 and breast cancer.