Hereditary renal carcinoma in the Eker rat is an excellent example of predisposition to a specific cancer being transmitted as a dominant trait. Recently, we identified a germline mutation of the tuberous sclerosis 2 (Tsc2) gene in the Eker rat. In the present study, we analyzed the upstream region of the Tsc2 gene. A novel leader exon (exon 1a) in a CpG island was found, and core promoter activity was identified in a 242-bp region of this island. Exon 1a and the promoter region were conserved in the human TSC2 gene. In addition, a rat homolog of a gene found upstream of TSC2 in human has been identified, indicating that the genomic organization around Tsc2/TSC2 is conserved between the two species. Characterization of the 5' region of Tsc2 and TSC2 will facilitate studies of the regulation of the gene and its disregulation in tumorigenesis.