sentences:  Mutations in the CFTR gene are responsible for causing cystic fibrosis.A particular mutation occurs in the CFTR pre-mRNA and leads to the exclusion of exon 9, mRNA lacking this exon folds a truncated protein (a protein shortened by a mutation).Exclusion of exon 9 is mediated by a polymorphic locus with variable TG repeats and stretches of T nucleotides – this is abbreviated as (TG)mT(n).