To gain insight into the molecular detail of MUC5B biosynthesis we investigated 1) the time course of MUC5B intracellular assembly, 2) the role of COOH-terminal protein domains in the assembly process, and 3) whether proteolytic processing was a feature of MUC5B assembly, packaging, or expansion in normal and CF cells. This evidence concerns the gene MUC5B and cystic fibrosis.