is an alternative name for sialic acid in the group of nonulosonic acids. 5‑N‑acetylneuraminic acid (Neu5Ac, purple diamond) is common in humans (Varki et al. 2015) and defines the group of sialylated, acidic human milk oligosaccharides. Numerous natural modifications are common and the substitutions on the hydroxyl groups affect recognition by proteins or – and often directly – its function (Varki et al. 2015).