The galactosemias are a series of three inborn errors of metabolism caused by deficiency of any one of the three human galactose-metabolic enzymes: galactokinase (GALK), galactose-1-phosphate uridyl transferase (GALT), and UDP-galactose 4' epimerase (GALE). We report here the characterization of the entire coding sequence of the GALE gene and screening for mutations in epimerase-deficient individuals. The human GALE gene is about 4 kb in size and is divided into 11 exons on chromosome band 1p36. We have identified five mutations in the GALE gene of epimerase-deficient galactosemia patients. The patients were either homozygotes or compound heterozygotes for mutations. These results confirm that epimerase-deficiency galactosemia is the result of missense mutations in the GALE gene and indicate that the disease is characterized by extensive allelic heterogeneity.