Gastric cancer is one of the most common malignancies in Japan. MUC6 present in gastric gland mucin carries O -glycans bearing α1,4-linked GlcNAc (αGlcNAc) at their nonreducing termini. αGlcNAc is synthesized by α1,4-N-acetylglucosaminyltransferase (α4GnT). Differentiated-type gastric adenocarcinoma spontaneously develops in A4gnt knockout (KO) mice and Muc6 KO mice, suggesting that both αGlcNAc and its scaffold protein MUC6 contribute to the regulation of gastric tumorigenesis. Moreover, cholesteryl α-D-glucopyranoside (CGL), a major component of the Helicobacter pylori (H. pylori)— cell wall, is essential for the microbe’s survival αGlcNAc has anti–H. pylori activity by inhibiting CGL biosynthesis. This chapter highlights the significance of αGlcNAc and MUC6 in gastric cancer. In this context, it also refers to cholestenone as a potential antibiotic against H. pylori through its inhibitory effects on CGL biosynthesis.