CXCR4 and neoplasm: Studies have shown that senescent tumor cells are enriched in the ' invasion front ' of PTC, and drive the collective invasion of CXCR4+ cancer cells by forming CXCL12 gradient, while improving the viability of tumor cells in lymphatic vessels and lymph nodes, revealing the active role of senescent cells in the metastasis cascade (Kim et al., 2017).