The gene responsible for cystic fibrosis, the most common severe autosomal recessive disorder, is located on the long arm of human chromosome 7, region q31-q32. The gene has recently been identified and shown to be approximately 250 kb in size. To understand the structure and to provide the basis for a systematic analysis of the disease-causing mutations in the gene, genomic DNA clones spanning different regions of the previously reported cDNA were isolated and used to determine the coding regions and sequences of intron/exon boundaries. A total of 22,708 bp of sequence, accounting for approximately 10% of the entire gene, was obtained. Alignment of the genomic DNA sequence with the cDNA sequence showed perfect colinearity between the two and a total of 27 exons, each flanked by consensus splice signals. A number of repetitive elements, including the Alu and Kpn families and simple repeats, such as (GT)17, (GATT)7, and (TA)14, were detected in close vicinity of some of the intron/exon boundaries. At least three of the simple repeats were found to be polymorphic in the population. Although an internal amino acid sequence homology could be detected between the two halves of the predicted polypeptide, especially in the regions of the two putative nucleotide-binding folds (NBF1 and NBF2), the lack of alignment of the nucleotide sequence as well as the different positions of the exon/intron boundaries does not seem to support the hypothesis of a recent gene duplication event. To facilitate detection of mutations by direct sequence analysis of genomic DNA, 28 sets of oligonucleotide primers were designed and tested for their ability to amplify individual exons and the immediately flanking sequences in the introns.