Galactosemia is a toxic clinical metabolic syndrome with increased blood galactose. Inborn defects in any of the three enzymes involved in galactose metabolism can cause galactosemia: ① Galactose-1-phosphate uridylyltransferase (Gal-1-PUT) deficiency: This is classic galactosemia and is relatively common; ② Galactokinase deficiency: It is relatively rare; ③ Uridine diphosphate galactose-4-epimerase (UDP-Gal-4-E) deficiency: It is rare. Galactosemia is an autosomal recessive congenital metabolic disease. In heterozygotes, the activities of the three related enzymes of galactose metabolism are about 1/2 of those of normal people, while in homozygotes, the enzyme activities are significantly reduced. The gene loci that control the three enzymes have been clarified. Uridyltransferase is on the short arm of chromosome 9, galactokinase is on the long arm of chromosome 17, and galactose-epimerase is on chromosome 1.