CXCR4 and leukemia: Indeed, direct leukemia-MSC contact via VCAM-1/VLA-4 interactions [82], together with CXCL12/CXCR4 signaling and inflammatory cytokine signaling [131,132], may contribute, to a different extent, based on space and time, to enhance leukemia survival by activating NF-kB signaling within the leukemic BM niche.