MUC5AC and cystic fibrosis: In solution, mucins interact both with one another and with other molecules such as calcium, which plays an important role in mucin cross‐linking.28, 29 Sequence variations in the central repetitive exon of the MUC5AC gene have been associated with severity of CF lung disease,30 perhaps resulting in MUC5AC polymers that have increased capacity to form cross‐links that will influence mucus function in terms of barrier porosity, viscosity, and transport by cilia.31