Galactosaemia is an inborn error of metabolism resulting in impaired activity of one of the four enzymes involved in the main galactose metabolism pathway (known as the Leloir pathway): galactokinase (GALK), galactose-1-phosphate uridylyltransferase (GALT), UDP galactose 4-epimerase (GALE) and the recently described galactose mutarotase (GALM) deficiency, (Figure 1a). The gene discussed is GALT; the disease is galactosemia.