MUC1 and MUC-X, epithelial mucins of breast and milk.

While reasons for gross variations in milk composition of the various mammalian species are well established, we will be revealing minor, subtle, yet highly important differences for years to come. One of the distinctive differences of human milk concerns its mucins, MUC1 and MUC-X. These are high molecular mass glycoproteins that occur on the apical surface of the lactating cell and are transferred to the milk fat globule upon its secretion from the cell. Among mammals, it seems that the human has developed the largest, most extended versions of these mucin molecules. Evidence is accumulating that this greater size benefits the human in terms of protection against infections and injurious environmental agents.