A direct link exists between malignant cells in ER+ breast cancers and breast adipose fibroblasts (BAFs) representing cancer-associated fibroblasts (CAFs) derived from preadipocytes: namely, that paracrine interactions of tumor cells and BAFs are essential for estrogen biosynthesis by the enzyme cytochrome P-450 aromatase (gene CYP19A1, chromosome 15q21.2) in BAFs [5,6]. The gene discussed is ESR1; the disease is breast cancer.