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 almost entirely absent from the cell membrane, and these critical ion transport functions are not performed. Having a homozygous pair of genes with the ΔF508 mutation prevents the CFTR protein from assuming its normal position in the cell membrane. This causes increased water retention in cells, corresponding dehydration of the extracellular space, and an associated cascade of effects on various parts of the body. These effects include: thicker mucous membranes in the epithelia of afflicted organs; obstruction of narrow respiratory airways as a result of thicker mucous and inhibition of the free movement of muco cilia; congenital absence of the vas deferens due to increased mucus thickness during fetal development; pancreatic insufficiency due to blockage of the pancreatic duct with mucus; and increased risk of respiratory infection due to build-up of thick, nutrient-rich mucus where bacteria thrive. These are the symptoms of cystic fibrosis, a genetic disorder; however, ΔF508 is not the only mutation that causes this disorder. Being a heterozygous carrier (having a single copy of ΔF508) results in decreased water loss during diarrhea because malfunctioning or absent CFTR proteins cannot maintain stable ion gradients across cell membranes. Typicallnucleotide-binding-up of both Cl− and Na+ ions inside affected cells, creating a hypotonic solution outside the cells and causing water to diffuse into the cells by osmosis. Several studies indicate that heterozygous carriers are at increased risk for various symptoms. For example, it has been shown that heterozygosity for cystic fibrosis is associated with increased airway reactivity, and heterozygotes may be at risk for poor pulmonary function. Heterozygotes with wheeze have been shown to be at higher risk for poor pulmonary function or development and progression of chronic obstructive lung disease. One gene for cystic fibrosis is sufficient to produce mild lung abnormalities even in the absence of infection. Mechanism The CFTR gene is located on the long arm of chromosome 7, at position q31.2, and ultimately codes for a sequence of 1,480 amino acids. Normally, the three DNA base pairs A-T-C (paired with T-A-G on the opposite strand) at the gene's 507th position form the template for the mRNA codon A-U-C for isoleucine, while the three DNA base pairs T-T-T (paired with A-A-A) at the