Elimination of both alleles of Mst1 together with heterozygosity for Mst2, and vice versa, results in the development of spontaneous hepatocellular carcinomas associated with loss of the remaining wild-type Mst1 or Mst2 allele in the tumors, whereas no tumors were observed in other organs of these mice. The gene discussed is MST1; the disease is hepatocellular carcinoma.