Isoelectric focusing of serum in cystic fibrosis: failure to distinguish between homozygote and heterozygote sera

Thin-layer isoelectric focusing was performed on samples of sera from patients with Cystic Fibrosis, siblings and obligate heterozygotes (parents), and children without cystic fibrosis (controls). The protein band with an isoelectric point of pH 5.48, previously reported to be absent in homozygote cystic fibrosis sera, was found to have a pI of 5.25. It was present in approximately one-half of the homozygote and heterozygote sera tested, and absent from 18 percent of control sera. The presence of this band is not therefore a reliable marker for the normal gene, and cannot be used to identify the heterozygous carrier for cystic fibrosis.