These results provide a better understanding of the mechanisms of the extramedullary dissemination and aid in the development of ALL therapies that target the extramedullary niche. To confirm whether LDN-193189 leukemic cells are proliferating after settling in the liver, we performed cell cycle analysis of leukemic cells harvested from the liver, PB, and BM. The liver and BM contained a substantial fraction of proliferating leukemic cells , while leukemic cells in the PB were predominantly non-proliferating. In vivo immunohistochemical staining with BrdU also showed that human CD45- positive leukemic cells in the liver were in the proliferation phase . Interestingly, significantly increased number of BrdU positive leukemic cells were present in the portal areas compared to that in the sinusoidal areas . Together, these results suggest that leukemic cells surrounding the bile ducts are not only anchored by bile duct epithelial cells, but also undergo proliferation around the bile duct epithelial cells. Previous studies have demonstrated that several adhesion molecules and chemokines, including CXCR4, CD56, CD44, CCR7, and VLA-4, contribute to the harboring and proliferation of leukemic cells in the BM niche and may enhance nichemediated resistance in leukemia . To identify molecules involved in the pathology of the ALL leukemic liver, we BAY-60-7550 PDE inhibitor analyzed their expression on leukemic cells harvested from the BM, liver, and spleen, in the three cases listed in Table 1. Flow cytometric analysis revealed that the percentage of CXCR4- positive leukemic cells was significantly higher in the liver than that in other organs . Consistent with previous studies , SDF-1 was expressed on bile duct epithelial cells in the portal area of the h-leukemic NOG model . Immunohistochemical analysis showed that CXCR4 positive leukemic cells were mainly distributed in the portal area, surrounding SDF-1 positive bile duct epithelial cells, but not in the sinusoidal area . Moreover, proximity ligation assay confirmed the specific interaction between CXCR4 on leukemic cells and SDF1 . Therefore, these results strongly suggest that the SDF-1/CXCR4 axis plays a role in the hepatic niche of leukemic cells. The SDF-1/CXCR4 axis is a key factor in the migration and proliferation of various cells, including neoplastic cells in vivo . Thus, we sought to directly examine the influence of the SDF-1/CXCR4 axis on leukemic cell migration and proliferation. First, we performed a chemotaxis assay by stimulating CXCR4 with its ligand SDF-1 . Leukemic cells harvested from the liver migrated avidly in response to SDF-1, and this migration was suppressed in the presence of AMD3100, a bicyclam molecule that antagonizes the binding of SDF-1 to CXCR4.