Galactosemia is an inherited disorder caused by the body's inability to break down galactose, a simple sugar found in milk, breast milk and other foods. Classic galactosemia, the most common and severe form of this disorder, is caused by deficiency of the galactose-1-phosphate uridyltranferase (GALT) enzyme. Persons with this disorder have too much galactose in the blood, which causes life-threatening complications including brain damage, liver enlargement, kidney failure, and cataracts. Treatment for galactosemia is best begun within the first days of life. If infants with classic galactosemia are treated promptly with a low-galactose diet, most of the complications can be prevented. Patients with galactosemia have a normal lifespan with proper treatment and diet restriction, although they are still at risk for cataracts, developmental delay, speech problems and premature ovarian insufficiency in women. Classic galactosemia is caused by pathogenic variants in the GALT gene.