ESR1 and cancer: The ability of CAFs to regulate responsiveness to endocrine therapy was further supported by the fact that low expression of a gene signature composed of the most highly downregulated genes (by fold change, cutoff >1.5) by CAF2 in MCF7 cells served as a predictive biomarker for poor response to treatment in patients with ER-α+ tumors in two different patient cohorts (Fig. 5B, C), again indicating suppression of hormonal pathways in luminal cancers by CAFs.