CFTR and cystic fibrosis: [3] The most common mutation, ΔF508, occurs in ∼70% of CFTR alleles. [1] Although numerous mutations in the CF gene have been described, there is little correlation between the specific mutation in the CF gene and severity of pulmonary disease. Recent investigations have focused on the identification of other non-CF gene variants that may affect the phenotype, i.e. “modifier genes.” [4], [5]