ERBB2 and breast cancer: These results show that even though well-established breast cancer cell lines express specific stem/progenitor cell-like protein markers, the 10AT-Her2 cell population can be considered to have a more enhanced ‘stemness’ character because of its highly efficient tumorsphere-formation property (Figure 2B), and as discussed in later sections, its ability to form tumor xenografts at low cell numbers in athymic mice.