The CFTR protein is largely expressed in cells of the pancreas, intestinal and respiratory epithelia, and all exocrine glands. When properly folded, it is shuttled to the cell membrane, where it becomes a transmembrane protein responsible for opening channels which release chloride ions out of cells; it also simultaneously inhibits the uptake of sodium ions by another channel protein. Both of these functions help to maintain an ion gradient that causes osmosis to draw water out of the cells. The ΔF508 mutation leads to the misfolding of CFTR and its eventual degradation in the ER. In organisms with two complements of the mutation, the protein is entirely absent from the cell membrane, and these critical ion transport functions are not performed.