Polycystic kidney disease (PKD) in the domestic cat results from a mutation in the gene PKD1. Knowing a cat has this gene does not tell a veterinarian how severe the disease will become or how long the animal will live a comfortable life, however, because other genes influence the progression of the disease. Several of the other genes that contribute to the proliferation of the disease have been identified in humans. These genes have yet to be investigated in the cat. This study will identify mutations that are correlated with severe disease progression. Knowing which genes are affected may impact the course of therapy and will aid in decisions regarding treatment of cats with polycystic kidney disease.