CXCR4 and leukemia: We and others have demonstrated that signaling between leukemia cells and stromal cells in the bone marrow microenvironment contributes to leukemia cell growth and survival, and that co-culture with normal human bone marrow stroma mediates therapeutic resistance.[1-10] The association between the cell surface receptor CXCR4 and the chemokine SDF-1 is thought to be an essential component of these interactions.