Although information on the correlation between patients’ genotype and phenotype is limited, clinical symptoms usually reflect the degree to which CFTR protein function is lost. Clinical presentation of CF, particularly pulmonary symptoms, varies widely, even among patients who have the same CFTR gene mutations. These variations suggest that environmental factors such as medical treatment and adherence to prescribed recommendations, or genetic factors such as modifier genes (non-CFTR alleles that can affect CFTR function), may play a significant role in symptom manifestation and disease progression.