ERBB2 and breast cancer: However, as (i) Zhan et al. have shown that only EGFR/ErbB2 heterodimers increased the invasive potential of mammary epithelial cells which is not observed with homodimers (Zhan et al., 2006), and as (ii) Guy et al. (1992) described that EGFR overexpression in mice was not associated with transformation of the entire mammary epithelium, but provoked only focal mammary tumors (sometimes metastatic), these results suggest that additional mechanisms are probably involved in ErbB activation effects on EMT and cell invasion.