Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Cystic Fibrosis ?
    ### Response:
    A defect in the CFTR gene causes cystic fibrosis (CF). This gene makes a protein that controls the movement of salt and water in and out of your body's cells. In people who have CF, the gene makes a protein that doesn't work well. This causes thick, sticky mucus and very salty sweat.
    