CXCR4 and neoplasm: In view of so far non-existing data on the role of CXCR7 in interaction with the reported tumor and metastasis-promoting actions of the CXCL12/CXCR4 axis in OS [9,10], we investigated in the present study the biological relevance of CXCR7 in a metastasizing intratibial human xenograft OS model in SCID mice that makes use of the human CXCR4-expressing 143B cell line and closely mimics the human disease with metastasis to the lung.