CXCR4 and head and neck squamous cell carcinoma: CXCL12, which is involved in CXCR4-mediated recruitment of CD8+ T cell recruitment to the tumor microenvironment [24], was upregulated by ionizing radiation in MOC2 cells but not in LY2, SCC83, and CAL27 HNSCC cells (Figure 2A–D).