Additionally, cancer stem cells (CSCs), which have been hypothesized to significantly play a role in resistance to therapy (see also below), have been reported to have a strong correlation with the overexpression of HER2 in different breast cancer models (Korkaya et al., 2008; Ithimakin et al., 2013; Shah and Osipo, 2016). This evidence concerns the gene ERBB2 and cancer.