ESR1 and breast carcinoma: MCF-7 breast cancer epithelial cells, a commonly used and well characterized ‘in vitro’ model of the most frequent human breast cancer subtype (i.e., luminal A estrogen receptor-positive one), were co-cultured with conditioned medium (CM) derived from CAFs treated with sildenafil, tadalafil, and vardenafil and growth was assessed by soft agar anchorage-independent assay (Figure 2I, upper panel).