Neuraminic acids are the commonest sialic acids in nature. Most sialic acids on gangliosides share a core neuraminic acid (Neu) structure and are N-acylated at the C-5 position with either an N-acetyl or an N-glycolyl group (giving Neu5Ac or Neu5Gc, respectively). It was originally thought that unsubstituted glycosidically linked Neu did not occur in nature. However, there have been several reports suggesting its presence in gangliosides and more recently in mucin-type glycoproteins. The N- or O-substituted derivatives of neuraminic acid are collectively known as sialic acids, the predominant one being N-acetylneuraminic acid. The amino group bears either an acetyl or a glycolyl group. The hydroxyl substituents may vary considerably: acetyl, lactyl, methyl, sulfate and phosphate groups have been found. Sialic acids are found widely distributed in animal tissues. Sialic acid rich glycoproteins bind selectin in humans and other organisms. Cancer cells that can metastasize often have a lot of sialic acid rich glycoproteins. This helps these late stage cancer cells enter the blood stream.