DPP4 and neoplasm: To investigate the role of CD26 in tumor progression and in API-mediated anticancer activity in vivo, we established an orthotopic lung tumor-bearing model by transplanting luciferase-tagged cells, A549-mock-luciferase, A549-CD26-luciferase, or A549-shCD26-luciferase (sh-775 and sh-777), into NOD-SCID mice and allowed them to become established for 7 days before initiating treatment.