An aldohexose that occurs naturally in the D-form in lactose, cerebrosides, gangliosides, and mucoproteins. Deficiency of galactosyl-1-phosphate uridyltransferase (galactose-1-PHOSPHATE URIDYL-TRANSFERASE DEFICIENCY disease) causes an error in galactose metabolism called GALACTOSEMIA, resulting in elevations of galactose in the blood.