Why do certain mutations cause cystic fibrosis? cystic fibrosis is the most common lethal genetic disease in the united states. it strikes one out of every 2,500 people of european descent, though it is less common in other groups. cystic fibrosis is the result of mutations in the cystic fibrosis transmembrane conductance regulator (cftr) gene. over 1,800 different mutations have been identified that can cause cystic fibrosis. people who inherit two recessive cftr alleles have the disease. below is a partial mrna and amino acid sequence for the wild-type cftr protein?