NNMT and neoplasm: In some of the breast cancer tissues included in this study, we noticed that the stroma cells were also stained positive for NNMT (Figure S7, Supporting Information), which is consistent with previous studies in colon cancer and ovarian cancer.[21, 30] From this aspect, both NNMT‐positive stroma cells and tumor cells are considered as main cellular source of 1‐MNA present in TME, which then serve as a signaling molecule transducing UBC12‐stabilizing signal from NNMT‐competent cells to adjacent NNMT‐free cells.