KITLG and neoplasm: Soluble tumor-derived factors responsible for the expansion of MDSCs include granulocyte-macrophage colony-stimulating factor (GM-CSF), granulocyte CSF (G-CSF), macrophage CSF (M-CSF), stem cell factor (SCF), vascular endothelial growth factor (VEGF), polyunsaturated fatty acids and semaphorin 4D [110,111].