Carcinogenesis is a multistep process in which cells accumulate multiple genetic alterations as they progress to a more malignant phenotype. Allelic chromosomal losses as well as mutations of tumor suppressor genes are common genetic events in the development and/or progression of both hereditary and non-hereditary tumors. Recent studies have shown a direct correlation between malignancy and inactivation of tumor suppressor genes for human cancers such as colorectal, hepatocellular and lung carcinomas. Although it is supposed that there are more than twenty tumor suppressor genes in the human genome, only a few tumor suppressor genes have been identified to date. Thus, further studies should focus on the identification and characterization of novel tumor suppressor genes, and molecular analyses of those genes in human cancer would be of great help in clarifying the multiple steps in the process of human carcinogenesis.