The major findings of our investigation are: i) patients affected by gastric cancer have significantly lower serum CD26 levels compared with healthy subjects ii) CD26 serum levels are associated with gastric cancer presence iii) CD26 measurement shows higher diagnostic efficiency compared CEA and CA19.9 in gastric cancer iiii) patients with HER2 positive tumors have significantly lower serum CD26 levels compared with the HER2 negative counterpart. The gene discussed is DPP4; the disease is gastric cancer.