Over-activation of Wnt-signaling and acidic extracellular pH correlate with cancer initiation/progression in patients and model organisms. Gastric cancer is one of the most common causes for cancer-related deaths. The gastric proton pump ATP4 acidifies the gastric lumen. Recently we have demonstrated that ATP4 is required for Wnt-signaling activation in Xenopus. Previous work by other groups have shown that inhibition of ATP4 induced selective apoptosis in human gastric cancer cells. My work will address how ATP4 and low pH act during Wnt-signaling activation and in gastric cancer.