Mucopolysaccharides are also known as GAGs. Uronic acid is either gluconic acid or its epimer L-iduronic acid; an exception is the keratin sulphate where sulphate galactose is present instead of acidic sugar. The amino sugar is either D-glucosamine or D-galactosamine, in which the amino group is usually acetylated. The amino sugar may be sulphated in the C-4 or C-6 position.