ESR1 and breast cancer: The BMP-antagonist, Gremlin 1 or GREM1, is closely linked with metastasis, specifically stemness of breast cancer cells (Ren et al., 2019), and accompanies the poor prognosis of patients with estrogen receptor-negative breast cancer (Neckmann et al., 2019), suggesting that Gremlin stands as a potent therapy for abrogating cancer cell (i.e., CSC) proliferation.