The aim of this study was to determine the CIN level and CH by using fluorescence in situ hybridization (FISH), in five human BC cell lines with differential expression of ERα and HER2 and to examine the association with the response to individual treatments, tamoxifen (TAM), docetaxel (DOC), doxorubicin (DOX), and Herceptin (HT), and combined treatments, TAM+DOC, TAM+DOX, TAM+HT, HT+DOC, and HT+DOX. This evidence concerns the gene ESR1 and breast cancer.