Tumor suppressor genes of Drosophila are developmental genes which, in the homozygously mutated state, induce in one step malignant or benign neoplastic transformation of specific cell types. They act early in development and by this set the stage for cell specific differentiation of imaginal discs, adult optic neuroblasts, blood and gonial cells. The structure, expression and possible function of the following four tumor suppressor genes are discussed: tumorous imaginal disc, lethal (3) malignant brain tumor, lethal (3) malignant blood neoplasm-1 and benign (2) gonial cell neoplasm.