GALT (Galactose 1 phosphate uridyl transferase) catalyzes the second step of the Leloir pathway of galactose metabolism, namely the conversion of UDP glucose + galactose 1 phosphate to glucose 1 phosphate + UDP galactose. The absence of this enzyme results in classic galactosemia in humans and can be fatal in the newborn period if lactose is not removed from the diet. The pathophysiology of galactosemia has not been clearly defined.