Illustration of closed cystic fibrosis transmembrane conductance regulator (CFTR) proteins (blue) on the membrane (orange) of a lung epithelial cell. The cell interior is top and the exterior is below. CFTRs are proteins responsible for the movement of negatively-charged ions, such as chloride (Cl-) and bicarbonate (HCO3-), in and out of cells. Genetic mutations can cause CFTRs to form incorrectly and remain closed, or Photographyt form at all, causing the condition cystic fibrosis. When CFTRs can Photographyt open, ions accumulate within cells. Water is drawn to areas of concentrated ions via osmosis, so a lack of ions outside cells reduces the amount of water around them. Epithelial cells in the lungs excrete mucus, and without sufficient water this becomes thick, sticky and difficult for the body to remove. This can cause difficulty breathing and increase the risk of lung infection.