They are broadly associated with gastric cancer progression, such as, by targeting SMAD7 and activating the TGF-/Smad pathway, exosomal miR-21-5p promotes mesothelial-to-mesenchymal transition (MMT) in peritoneal mesothelial cells (PMCs) and peritoneal metastasis of gastric cancer (31). Here, SMAD7 is linked to gastric cancer.