CXCR4 and neuroblastoma: It is worth noting there was one cell line, NGP, in which some SDF-1 was detected via ELISA (Fig. 5), however the maximum concentration detected (184 pg/ml) was more than an order of magnitude below the minimum concentration of exogenous SDF-1 (10 ng/ml) used to mediate CXCR4 down-regulation in neuroblastoma cells [18].