CFTR and cystic fibrosis: However, in approximately 85% of patients with CF, at least one allele of the CFTR gene is affected by the most common mutation c.1521_1523del, resulting in the deletion of p.Phe508 (NM_000492.3: c.1521_1523del, hereafter referred to as p.Phe508del, dbSNP: rs113993960).