ESR1 and breast cancer: To explore the role of STAT1 in breast tumor development, the relative cellular levels of STAT1 protein were immunohistochemically assessed in a cohort of 161 primary breast cancer samples (78 ERα- and 83 ERα+ cases) (Table 1) by using a STAT1-specific polyclonal IgG antibody that recognizes a STAT1 epitope shared by the human and mouse proteins (Figure 1A).