Galactose metabolism in all organisms is catalyzed by three enzymatic steps: the galactokinase, galactose-1-phosphate uridyltransferase, and UDP galactose 4'-epimerase reactions. We report here the molecular cloning, characterization, and mapping of a full-length cDNA encoding human UDP-galactose 4'-epimerase (GALE). Our cDNA is 1488 bp long and matches the mRNA size of 1.5 kg detected in fibroblasts and lymphoblasts. The human GALE cDNA encodes a predicted protein of 348 amino acids with a molecular mass of 38,266. The human GALE enzyme is 87% identical to the rat protein, 53% identical to the homologous GAL10 protein from the yeast Kluyveromyces lactis, and 51% identical to the galE protein from the prokaryote Escherichia coli. This extraordinary degree of sequence identity has allowed us to build a homology model of the human protein based on the bacterial crystal structure. This predicted human structure is very similar to the E. coli galE enzyme, suggesting that both enzymes use similar mechanisms. The human gene encoding GALE maps, as expected, to a single locus on chromosome 1 and appears to be compact. The human GALE gene is structurally intact in 19 patients with epimerase-deficiency galactosemia, an inborn error of metabolism secondary to GALE deficiency. Therefore, we propose that this disorder is due to small mutations within the gene.