The SLIT1 cDNA encodes a 1,534-amino acid polypeptide with 43.5% similarity to the Drosophila 'slit' protein. Northern blot analysis revealed that the human SLIT1 gene was expressed as a major 8.4- and a minor 5.9-kb transcript primarily in the brain. SLIT2 and SLIT3 mRNAs were primarily expressed in the spinal cord and thyroid, respectively. In situ hybridization studies indicated that the rat Slit1 mRNA was specifically expressed in the neurons of fetal and adult forebrains. These data suggested that the SLIT genes form an evolutionarily conserved group in vertebrates and invertebrates, and that the mammalian SLIT proteins may participate in the formation and maintenance of the nervous and endocrine systems by protein-protein interactions.