Classic galactosemia is an inborn error of metabolism caused by deficiency of the enzyme galactose 1-phosphate uridyl transferase (GALT) in the Leloir pathway of galactose metabolism (Fig. 1), which includes three enzymes: galactokinase (GALK), galactose 1-phosphate uridyltransferase (GALT) and UDP-galactose 4-epimerase (GALE). The gene discussed is GALK1; the disease is classic galactosemia.