In studies of the effect of of Zebularine on the antigenicity of tumor cells [8], we found that treatment with a moderately high dose of Zebularine (100 μM) increased the expression of the gene of the enzyme indolamine-2,3-deoxygenase-1 (IDO1) in rat colon cancer cells in vitro and drastically reduced their immunogenicity. The gene discussed is IDO1; the disease is neoplasm.