Cystic fibrosis is not caused by a defect in the gene coding for human complement C3.

A human genomic clone containing a portion of the structural gene for the third component of complement (C3) was used as a hybridization probe to DNA from two genetically informative families in which cystic fibrosis (CF) occurs. Several restriction fragment length polymorphisms (RFLPs) can be identified using the C3 probe, which appears to detect a high level of sequence variation in the general population. The inheritance of C3 RFLP was found to be independent of the inheritance of the CF phenotype. Assuming that CF is caused by a mutation affecting a single genetic locus, the demonstration that an allele of the C3 gene does not segregate with CF proves that a defect of complement C3 cannot be the cause of the disease.