The tumor suppressor gene, lethal(3)malignant blood neoplasm-1+, of Drosophila melanogaster is required for the differentiation of the phagocytic blood-cell type, the plasmatocyte. In the homozygously mutated state it causes the malignant transformation of these blood cells. We present here the cloning, sequencing, structure, and expression of the l(3)mbn-1+ gene during development. The cloned gene was identified by germ-line transformation, generation of revertants, and the detection of the corresponding mRNA in blood cells and other tissues. Homologies of the G-S-rich C-terminus of the putative MBN83 protein to human cytokeratins K1, K10, and mouse loricrin were found. The structure and possible function of the wild-type l(3)mbn-1+ gene are discussed.