This review article focuses on the multi-domain organization of the five human gel-forming mucins MUC2, MUC5B, MUC5AC, MUC6 and MUC19. Gel- forming mucins are O-glycoproteins with high molecular mass that are secreted at the cell surface of many secretory organs by specialized epithelial cells. They form dimers through their carboxy-termini regions and then multimers through their amino- termini regions before being secreted into the lumen of organs as long linear filaments. In addition, an overview of their genomic organization is given and we took the opportunity of this article to determine the genomic organisation of the 5'-end region of MUC5AC. Finally, we discuss the polymorphism of mucins and a few features of long messengers.