The locus responsible for the most common form of autosomal dominant polycystic kidney disease (PKD1) is located on chromosome 16p13.3. Genetic mapping studies indicate that PKD1 is flanked on the proximal side by the DNA marker 26.6 (D16S125). Here we show that 26.6 has undergone a locus duplication and that the two loci are less than 150kb apart. One of the two loci contains a polymorphic TaqI site that has been used in genetic studies and represents the proximal boundary for the PKD1 locus. We demonstrate that the polymorphic locus is the more proximal of the two 26.6-hybridizing loci. Therefore, four cosmids isolated from the distal 26.6-hybridizing locus contain candidate sequences for the PKD1 gene. These cosmids were found to contain two CpG islands that are likely markers for transcribed regions. A third CpG island was detected and cloned by directional chromosome jumping.