A recent report demonstrated profound differences in the makeup of tumor associated immune cells for metastatic tumors from ER mutants compared with those from wt ER, including higher levels of regulatory and helper T cells and immune-suppressive macrophages10, raising the possibility that the elevated interferon signaling proteins identified here have an immunosuppressive role in aiding the metastatic potential of ER mutant breast cancer. This evidence concerns the gene ESR1 and breast cancer.