ERBB2 and breast cancer: Furthermore, several studies have highlighted that HER2 regulates the stem cell population and then contributes to mammary carcinogenesis (Bedard et al., 2009) and that HER2 overexpression in multiple breast cancer cell lines results in an increase of ALDH1+ cell fraction, which has a greater capacity to invade and form tumors in immunodeficient mice (Korkaya et al., 2008).