GDNF and cancer: In addition, the main chemokines include chemokine CCL2 and matrix-derived factor 1 (SDF-1/CXCL12), which induce cancer cell migration under the action of CCR2 and CXCR4 receptors respectively, and can also recruit bone marrow-derived cells (BMDC) and M2 macrophages, and the recruited macrophages secrete GDNF, which can activate RET-GDNF receptor α1 (GFRα1) in cancer cells to promote PNI and the invasion of cancer cells [30,143].