As a downstream transcription factor of the TGF-β superfamily, SMAD4 can negatively regulate the expression of CCL15 by binding to the promoter of CCL1 5[29], and CCL15 is upregulated in SMAD4-deficient colorectal cancer, and in which mediates the recruitment of MDSC through a CCL15/CCR1-dependent manner [20]. Here, CCL15 is linked to colorectal cancer.