STAT3 and cancer: Two functional proteins, such as CCAAT/enhancer-binding protein (c/EBPβ) and STAT3, which promote generation, differentiation, and expansion of MDSCs, despite are not surface markers, could help to define two different MDSCs subgroups (CD11b+HLA-DR−c/EBPβ+ and CD33+HLA-DR-STAT3+) and could provide new diagnostic and therapeutic tools for cancer immunotherapy (Lechner et al., 2011; Wu et al., 2011; Wang et al., 2019a).