Amino sugars are formed when the hydroxyl group is replaced by an —NH₂ group. Examples are glucosamine and galactosamine. Amino sugars occur as N-acetyls in numerous biological important compounds. N-acetyl derivative of D-glucosamine is present in hyaluronic acid, heparin, and some blood group compounds. Galactosamine is present as N-acetylgalactosamine in chondroitin sulphate. Antibiotics such as erythromycin and caromycin also contain amino sugars.