It is necessary to monitor gastrin levels in a few conditions, including (i) refractory or recurrent peptic ulcer disease (PUD) in the absence of non-steroidal anti-inflammatory drugs or helicobacter pylori (H. pylori) infection, (ii) PUD in unusual locations (e.g. beyond the duodenal bulb), (iii) PUD with concurrent endocrinopathies, (iv) gastroesophageal reflux disease (GERD) refractory to proton pump inhibitors (PPIs) and/or with distal esophageal strictures, (v) presence of prominent rugal folds seen on upper endoscopy, (vi) chronic secretory diarrhea and (vii) gastric carcinoids. This evidence concerns the gene GAST and gastroesophageal reflux disease.