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Cystic fibrosis is caused when an individual inherits two copies of a recessive gene that codes for a defective form of the protein CTFR. The defective protein results from the deletion of three bases that together code for the amino acid phenylalanine. Without the phenylalanine, the protein that is made does not fold properly. The body, sensing this defective protein, starts destroying the copies of the defective protein. This protein is important because it controls the passage of chloride ions into and out of the cell. If chloride ions cannot pass through, chloride ions accumulate where they shouldn't, causing the mucous in the respiratory system to be thicker than usual and to clog airways.