To further demonstrate the critical role of the proinflammatory cytokine secretion by breast cancer cells in SMAD3‐mediated tumor metastasis, we generated IL‐6 KO 4T1 cells (Figure S8A, Supporting Information) and found that IL‐6 KO inhibited SMAD3‐2KQ‐induced MDSCs recruitment (Figure S8B,C, Supporting Information). Here, IL6 is linked to breast cancer.