CCL21 and neoplasm: Soluble PD-1 had also shown enhancement of the anti-tumor effects induced by other gene-therapeutic agents such as secondary lymphoid tissue chemokine (SLC, CCL21), Herpes Simplex Type 1 Thymidine Kinase (TK) (HSVtk), a recombinant fibronectin (CH50), and 4-1BB ligand gene, mainly through strengthening CD8+ T cell responses resulting in tumor growth reduction (104, 115–118) (Figure 4).