Zhu et al. showed that GC cell-derived exosomal nicotinamide N-methyltransferase (NNMT), an S-adenosyl-L-methionine-dependent cytoplasmic enzyme, promoted the malignancy of HMrSV5 cells, an established human peritoneal mesothelial cell line, by activating TGF-β/Smad2 signaling [34]. Here, TGFB1 is linked to gastric cancer.