Research into inherited cancer syndromes that involve defective tumor suppressor genes has increased our understanding of the genetic basis of this disease. Dramatic advances over the past decade have established the tuberous sclerosis complex 2 tumor suppressor gene (TSC2) as a key player in signal transduction pathways involved in the development of cancer. Importantly, the discovery of the functional link between TSC2 and the polycystic kidney disease 1 gene (PKD1) is beginning to build a foundation for understanding the heritable diseases associated with defects in each of these genes, namely, tuberous sclerosis complex and polycystic kidney disease. In this review, we summarize the latest findings about the functions of the TSC2 gene product, tuberin, and their implications for the development of cystic kidney disease.