Although activated STAT3 is not only expressed in MDSCs, we will limit the remainder of this review to the role of STAT3 in these myeloid cells (Figure 1), as in contrast to other tumor-infiltrating immune cells like dendritic cells (DCs), macrophages and T cells, MDSCs are abundantly present in most mouse tumor models and human cancers [26], [31]. The gene discussed is STAT3; the disease is cancer.