Based on GALT residual activity, galactosemia can be further classified into three clinical/biochemical phenotypes: (i) classic galactosemia (CG), which is characterized by the absence of GALT activity in erythrocytes and liver, (ii) clinical variant galactosemia, which is associated with a drastic reduction in GALT activity (1–10% residual activity in erythrocytes and/or liver), and (iii) biochemical variant galactosemia, which shows 15–33% residual GALT activity in erythrocytes [3]. The gene discussed is GALT; the disease is galactosemia.