Ghrelin plays an essential role in the mechanism of gastric mucosal defense, the reduction of ghrelin production in the fundic is the main reason for the decrease of serum ghrelin concentration, and it is closely related to the ratio of PG I/PG II, in which the PG I/PG II < 3 is considered a reliable sign of severe atrophic gastritis. The gene discussed is GHRL; the disease is chronic atrophic gastritis.