Approximately what proportion of cystic fibrosis cases is accounted for by the most common CFTR mutant allele?

• A. About 30%
• B. About 50%
• C. About 70%
• D. About 90%

Answer: (C)

Cystic fibrosis is caused by mutations in the CFTR gene, and one specific mutation—ΔF508—is far more common than all others. This deletion removes a phenylalanine at position 508, causing the CFTR protein to misfold and be degraded rather than reaching the cell surface, which greatly impairs chloride transport. Because this single mutation is so prevalent in many populations (especially those of European descent), it accounts for about 70% of CF cases overall. That means the majority of individuals with CF carry this mutation either as two copies (homozygous ΔF508) or with one ΔF508 allele and another CFTR mutation (compound heterozygote). The remaining cases come from a mix of many rarer mutations.