As transforming growth factor (TGF-β), a well-known regulator of the cancer–stroma interaction, was reported to regulate miR21 expression in pulmonary artery smooth muscle cells20 and pulmonary fibroblasts21, we determined whether increased miR21 expression in ovarian cancer cells was due to TGF-β produced by the stromal cells, which upregulated endogenous miR21 in ovarian cancer cells rather than exosomal transfer. Here, TGFB1 is linked to ovarian cancer.