CXCR4 and neuroblastoma: Furthermore, the apparent lack of endogenous SDF-1 involvement in our studies demonstrate that regulation of CXCR4 surface expression can occur in a non-autocrine, SDF-1 independent manner, which suggests that this chemokine is not the sole obligatory signal for all CXCR4 mediated activity, a likelihood which was first underscored in a study using neuroblastoma cells that were metastatically active and positive for CXCR4 surface expression yet desensitized to SDF-1 [19].