Using a positional cloning approach the major autosomal dominant polycystic kidney disease (ADPKD) gene (PKD1) has been identified on chromosome 16: a disease associated chromosome translocation was instrumental in its identification. Study of the PKD1 gene has been complicated because most of the gene lies in a genomic region reiterated elsewhere on the same chromosome. The duplicate area contains three genes which share substantial homology with PKD1 and generate polyadenylated transcripts. Most PKD1 mutations have so far been detected in the single copy, 3' end of the gene, but a group of patients with deletion of PKD1 and the adjacent TSC2 gene, which have severe infantile polycystic kidney disease, have also been characterised. The full length transcript of PKD1 (approximately 14 kb) has now been cloned and is predicted to encode a protein, polycystin, of 4302/3 aa. Polycystin contains multiple extracellular domains including leucine rich repeats, a C-type lectin, immunoglobulin and fibronectin type III-like domains and has a C terminal region which is likely associated with the membrane. These homologies indicate that polycystin is a cell-cell/matrix interaction protein.