Four main enzymes catalyze a specific reaction in the Leloir pathway (Figure 1): galactose mutarotase (GALM, EC 5.1.3.3), galactokinase (GALK1, EC 2.7.1.6), galactose-1-phosphate uridylyltransferase (GALT, EC 2.7.7.12), and UDP-galactose 4′-epimerase (GALE, EC 5.1.3.2) Thus, galactosemia, can be classified into four different forms (1–4) according to the altered galactose metabolic enzyme: (i) type 1 galactosemia; (ii) type 2 galactosemia; (iii) type 3 galactosemia; (iv) type 4 galactosemia [5]. The gene discussed is GALK1; the disease is classic galactosemia.