We have cloned a rat homologue of the Drosophila recessive oncogene lethal (2) giant larvae from rat brain by RT-PCR using primers prepared from sequences conserved amongst lgl family genes. Sequence analysis predicts that the rat rgl-1 gene encodes a 1,036 amino acid polypeptide with a molecular weight of approximately 112 kDa, which contains a domain characteristic of WD-40 proteins. Northern blot analysis revealed that the highest expression of rgl-1 is detected in the testis, with moderate expression in ovary, brain, spleen, and kidney. Since there is a high degree of amino acid similarity among lgl proteins in various species, it is likely that there is an evolutionary relationship among these proteins. The amino acid identity of rgl-1 to Drosophila l(2)gl and mouse mgl-1 proteins showed 30.6 and 96.8%, respectively. The rat tomosyn, previously known as a homologue of Drosophila l(2)gl, showed much lower amino acid identity to Drosophila l(2)gl and mouse mgl-1 proteins (17.8 and 20%, respectively). Functional analysis showed that the expression of a rat rgl-1 cDNA in Saccharomyces cerevisiae missing sop genes, the yeast homologues of the Drosophila l(2)gl, restored partially the Na+ tolerance of the cell. Taken together, these results indicate that rgl-1, not tomosyn, is the legitimate homologue of lgl gene.