Background The impact of complex alleles on CFTR processing and function has yet not been investigated in native human tissue. Methods Intestinal current measurements (ICM) followed by CFTR immunoblot were performed on rectal biopsies taken from two siblings who are compound heterozygous for the CFTR mutations p.Phe508del and the complex allele p.[Arg74Trp;Val201Met;Asp1270Asn]. Results Normal and subnormal chloride secretory responses in the ICM were associated with normal and fourfold reduced amounts of the mature glycoform band C CFTR, respectively, consistent with the unequal clinical phenotype of the siblings. Conclusion The combined use of bioassay and protein analysis is particularly meaningful to resolve the CFTR phenotype of “indeterminate” borderline CFTR genotypes on a case‐to‐case basis.

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