Employment of these genes enabled a clear discrimination between the two breast cancer cell lines BT-474 and MCF-7 with high and low HER2 expression levels, respectively, of the target gene ERBB2. Still, despite our extensive survey for the best references, an exclusion of samples from data analysis due to lack of expression of reference genes could not be avoided, thus challenging the use of the relative expression quantification for the analysis of rare cells such as DCCs. This evidence concerns the gene ERBB2 and breast cancer.