D-Galactose (CAS: 59-23-4) is an aldohexose that occurs naturally in the D-form in lactose, cerebrosides, gangliosides, and mucoproteins. D-Galactose is an energy-providing nutrient and also a necessary basic substrate for the biosynthesis of many macromolecules in the body. Metabolic pathways for D-galactose are important not only for the provision of these pathways but also for the prevention of D-galactose metabolite accumulation. The main source of D-galactose is lactose in the milk of mammals, but it can also be found in some fruits and vegetables. Utilization of D-galactose in all living cells is initiated by the phosphorylation of the hexose by the enzyme galactokinase (E.C. 2.7.1.6) (GALK) to form D-galactose-1-phosphate. In the presence of D-galactose-1-phosphate uridyltransferase (E.C. 2.7.7.12) (GALT) D-galactose-1-phosphate is exchanged with glucose-1-phosphate in UDP-glucose to form UDP-galactose. Glucose-1-phosphate will then enter the glycolytic pathway for energy production. Deficiency of the enzyme GALT in galactosemic patients leads to the accumulation of D-galactose-1-phosphate. Classic galactosemia, a term that denotes the presence of D-galactose in the blood, is the rare inborn error of D-galactose metabolism, diagnosed by the deficiency of the second enzyme of the D-galactose assimilation pathway, GALT, which, in turn, is caused by mutations at the GALT gene. Galactose in the urine is a biomarker for the consumption of milk.