Among tumor-related factors involved in the generation of MDSCs and their recruitment from bone marrow to tumor microenvironment are mediated by granulocyte–macrophage colony-stimulating factor (GM-CSF), IL-6, indoleamine 2,3-dioxygenase (IDO) and chemokines (e.g. CCL26, CXCL12, CXCL8). Here, IDO1 is linked to neoplasm.