Fetuin (NEB# P6042) or pNP-N-acetyl-neuraminic acid. Fetuin has both α2-3 and α2-6 terminal sialic acid residues; α2-3 Neuraminidase S will only cleave α2-3 linked residues, therefore the molecular weight change (due to sialic acid loss) will be intermediate if compared to treatment with a general neuraminidase.