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