Of the 28 presently known Drosophila tumor suppressor genes we present the status of the functional analysis of the following three genes: (a) lethal (3) malignant brain tumor [1(3)mbt], which by homology belongs to the Pc-G gene family and may be involved in the stable silencing of specific developmental genes by changing the chromatin structure, and thus establishing and maintaining the differentiated state; (b) lethal (3) malignant blood neoplasm-1 [1(3)mbn-1], for whose function only vague predictions can be made; 4) benign (2) gonial cell neoplasm [b(2)gcn], which may function as a splice factor. Each Drosophila tumor suppressor gene transforms in the homozygously mutated state either one or two specific cell-types in a single step, and is thus the primary cause for tumorigenesis. For one of the genes a putative human homologue has been found.