Tianyuan Ci et al. discovered that the bone marrow-homing and retention capabilities of leukemia cells are related to the expression of the chemokine receptors C-X-C chemokine receptor type 4 (CXCR4) and cluster of differentiation 44 (CD44) on their cell membranes [26] (Figure 2A). Here, CXCR4 is linked to leukemia.