GDF15 and neoplasm: The cytokines C‐C motif chemokine 2 (CCL2), macrophage colony‐stimulating factor 1 (CSF1), and growth differentiation factor 15 (GDF15) produced by tumor cells are the main regulators of macrophage infiltration and survival.[20] qPCR detection (Figure 3h–j) suggested that the mRNA expression of CCL2, CSF1, and GDF15 was dramatically decreased upon SLITRK4 knockdown, whereas SLITRK4 overexpression promoted their production.