A gene is provided which is present in the deletion region of a chromosome common in lung cancer, hepatocellular carcinoma and colorectal cancer and encodes a novel protein, a protein encoded by the gene (PRLTS protein), and a method of discriminating tumor cells. With respect to the human chromosome 8, a detailed gene map was prepared, the chromosome of each of lung cancer, hepatocellular carcinoma and colorectal cancer tissues was analyzed, and a gene encoding a novel protein was cloned to thereby determine the structure thereof. A gene analysis was conducted with the use of a DNA probe derived from the above gene, and consequently mutations in the gene were confirmed in the lung cancer, hepatocellular carcinoma and colorectal cancer tissues.