- Heterozygous with one allele represented as any CFTR mutation and the other allele must represent a class III, IV, V, VI CFTR mutation (Note: since the CFTR mutation, F508del, can be considered either a class II or III mutation, heterozygous CF patients that have one allele that contains F508del, must have the other allele contain a class III (i.e., not F508del), IV, V, or VI mutation). Patients with F508del/F508del mutation should only be included in Part 3 Cohort 3.