ESR1 and breast cancer: Although breast CAFs are morphologically homogeneous, gene expression profiles vary between invasive breast cancer subtypes.25 Stromal genetic profiling predicts prognosis independent of ER, HER2, grade, nodal status, and previous treatments.6 For example, fibroblasts with a “wound‐healing” genotype are predictors of metastases and death in breast cancer.26 Different CAF phenotypes are heterogeneous in their tumor‐promoting properties with, for example, HGF‐ and HSF1‐expressing,27, 28 and Caveolin‐1‐deficient29 fibroblasts all demonstrating increased tumor‐inducing ability.