Galactosemia is a group of rare genetic metabolic disorders characterized by impaired galactose metabolism resulting in a range of variable manifestations encompassing a severe, life-threatening disease (classic galactosemia), a rare mild form (galactokinase deficiency) causing cataract, and a very rare form with variable severity (galactose epimerase deficiency) resembling classic galactosemia in the severe form (see these terms).Overall prevalence is unknown. The annual incidence of classic galactosemia is estimated to be between 1/40,000 and 1/60,000 in Western countries. Incidence appears to be variable in other ethnic groups with a higher rate reported in the Irish Traveler population possibly due to consanguinity.Infants usually develop feeding difficulties, poor weight gain and growth, lethargy, and jaundice in the common severe form of the disorder, i.e. classic galactosemia. The rare, less severe clinical subtype of galactosemia (galactokinase deficiency) causes primarily cataracts while other signs of galactosemia are absent. The very rare subtype (galactose epimerase deficiency) has a variable clinical picture including the usual signs of galactosemia (hypotonia, poor feeding, vomiting, weight loss, jaundice) and complications such as impaired growth, cognitive deficit and cataracts.The different galactosemia types are caused by mutations in the GALT, GALK1, and GALE genes (9p13, 17q24, 1p36) coding for the three enzymes essential in galactose metabolism resulting in impairment of the Leloir galactose degradation metabolic pathway. All three diseases follow an autosomal recessive pattern of inheritance.