Background: Some people have a mutation in the cadherin-1 gene (CDH1) gene that is known to lead to stomach cancer. They are advised to get regular endoscopies with biopsies even if their stomach appears normal. The endoscopy method currently used is called the 'Cambridge Method.' Researchers want to test a new method called the 'Bethesda Protocol.'

Objective: To compare the Cambridge Method and Bethesda Protocol and find out which is more efficient in catching early signs of cancer.

Eligibility: Adults age 18 and older who have a mutation in the CDH1 gene.

Design: Participants will be screened with a review of their medical history, medical records, and physical status. Participants will be put into group 1 (Bethesda Protocol) or group 2 (Cambridge Method). Participants will have a physical exam. They will have endoscopy. For this, they will be put under general anesthesia. They will wear compression cuffs around their legs to prevent blood clots. A lighted tube will be inserted into their mouth and go down to their stomach. For group 1 participants, 88 pieces of tissue will be taken from 22 areas of their stomach. For group 2 participants, 30 pieces of tissue will be taken from 6 areas of their stomach. Then group 2 will be injected with a contrast dye. A microscope will be inserted, and more samples will be taken. About 14 days later, participants will have a follow-up visit or phone call. They may give stool samples every 3 to 6 months for 12 months for research purposes. Participants may have another endoscopy 6-18 months later.