S100A9 and acute myeloid leukemia: Mesenchymal stromal cells (MSCs), one of the most important niche-forming cells in the BM microenvironment, can communicate with AML cells through soluble factors (IL-6), which results in the upregulation of S100A8 and S100A9 expression (S100A8high and S100A9high) and activation of the JAK/STAT3 signaling pathway in AML cells (Figure 2A).