Figure 1.

Human milk oligosaccharide composition blueprint. HMO composition follows a basic blueprint shown in the center. HMOs can contain 5 different monosaccharides in different number and linkages, namely glucose (blue circle), galactose (yellow circle), N-acetlylactosamine (blue square), fucose (red triangle), and sialic acid (purple diamond). All HMOs carry lactose at the reducing end. Lactose can be fucosylated or sialylated to generate the small HMOs 2′-fucosyllactose and 3-fucosyllactose or 3′-sialyllactose and 6′-sialyllactose, respectively (upper left corner). Alternatively, lactose can be elongated with type 1 or type 2 disaccharide units to form linear or branched HMOs (upper right corner). Elongated HMOs then can be sialylated (lower left corner) or fucosylated (lower right corner) or both sialylated and fucosylated (not shown). The HMOs in this figure are only a few relatively simple examples. So far, more than 150 different HMO structures have been identified.