Galactosemia results from a hereditary defect of the galactose-1-phosphate uridyl transferase enzyme, which is necessary for proper metabolism of the sugar, galactose (Fig 1). This molecular disease provides a useful model for the consequences of mutations in humans which alter gene, function. Molecular disease, as a concept, is not new, but progress in understanding these inborn enzyme defects has been striking during the last decade.1 The advances made in the understanding of both the biochemistry and genetics of these hereditary diseases have resulted in improved medical practice.
Galactosemia is a molecular disease which exemplifies the classical model of an autosomal-recessive disorder.2,3 Because human erythrocytes reflect the pathways of metabolism of galactose, they have been a convenient source of tissue for biochemical and genetic analysis.4 By chemical manipulation of isolated erythrocytes the occurrence of galactosemia can be determined with certainty and heterozygotes or carriers from both the diseased