Schematic model of the molecular mechanism by which high levels of Hes1 expression promote the development of CML-BC. Hes1 expression is frequently upregulated in BCR-ABL⁺ leukemic cells, in which Hes1 upregulates MMP-9 through NF-κB activation. MMP-9 is secreted by leukemic cells and promotes migration and proliferation of leukemic cells that are in conjunction with stromal cells. In addition, MMP-9 sheds sKitL from stromal cells, and increased sKitL enhances proliferation of leukemic cells.