CXCR4 and neuroblastoma: One prominent signaling axis involves CXCR4 and its ligand CXCL12: Early research showed that NB cells express CXCR4, which seems to play a critical role in metastasis to the BM [105,106] and that the level of CXCR4 expression is correlated with BM metastasis and poor clinical outcome [107].