MMP9 and B-cell acute lymphoblastic leukemia: Matrix metalloproteinase-9 (MMP-9) produced by leukemia cells facilitates tumor progression via remodeling of the ECM of the BM microenvironment, and MMP-9 deficiency in the BM microenvironment reduces leukemia-initiating cells and prolongs survival of mice with BCR-ABL1-positive B-cell acute lymphoblastic leukemia (B-ALL)75.