Retinblastoma gene (Rb gene) for controlling susceptibility to retinoblastoma is located on human chromosome 13q14 spanning about 200Kb with 27 exons. The Rb gene transcripts as 4.7 kb mRNA and encodes a phosphorylated protein (Rb protein) of approximately 110 kd in molecular mass and accumulates in the nuclei of most vertebrate cells. Deletions or mutations of Rb gene that ablate normal gene expression have been found in all retinoblastoma examined to date, as well as many other malignant tumors such as osteosarcoma, carcinomas of breast, lung, prostate gland and liver, etc. According to these characteristics, we consider Rb gene an antioncogene or tumor suppressor gene. We have tested this hypothesis by introducing a Rb gene expression vector into the cells of breast cancer and hepatocellular carcinoma, and by introducing the antisense Rb gene into normal human cells. The Rb gene transcripts as 4.7 kb mRNA and encodes a phosphorylated protein (Rb protein) of approximately 110 kd in molecular mass and accumulates in the nuclei of most vertebrate cells. Deletions or mutations of Rb gene that ablate normal gene expression have been found in all retinoblastoma examined to date, as well as many other malignant tumors such as osteosarcoma, carcinomas of breast, lung, prostate gland and liver , etc. According to these characteristics, we consider Rb gene an antioncogene or tumor suppressor gene. We have tested this hypothesis by introducing a Rb gene expression vector into the cells of breast cancer and hepatocellular carcinoma, and by introducing the antisense Rb gene into normal human cells.