Gastric mucin is a large glycoprotein which is thought to play a major role in the protection of the gastrointestinal tract from acid, proteases, pathogenic microorganisms, and mechanical trauma. In this paper we describe the isolation by expression cloning and characterization of cDNAs which code for human gastric mucin. The cDNA sequence is characterized by a tandem repeat region whose individual repeat unit is 507 base pairs (169 amino acids) long. The translated sequence is rich in threonine, serine, and proline (31, 18, and 15%, respectively) and contains a relatively large amount of histidine (7.1%) and alanine (5.6%). RNA blot analysis shows a polydisperse pattern which is characteristic of mucins. Expression of this gene is highest in the stomach and gall bladder, with weaker expression in the terminal ileum and right colon. This expression pattern is different from other human mucins and indicates that this gene codes for a unique mucin. Fluorescence in situ hybridization techniques have localized this gene to chromosome 11p15.4-11p15.5. This is the third mucin to be localized to the 11p15 region and suggests a clustering of secretory mucin genes. We propose that this gene for human gastric mucin be called MUC6.