Diverse structures of sialic acids (SA). (A) Neuraminic acid; (NeuC); (B) N-acetyl neuraminic acid (NeuAc); (C) N-glycolyl neuraminic acid (NeuGc); (D) N, O-diacetyl neuraminic acid (occurs in horse); (E) N, O-diacetyl neuraminic acid (occurs in bovine); (F) N-acetyl O-diacetyl neuraminic acid (occurs in bovine).