Avidin is a tetrameric or dimeric biotin-binding protein produced in the oviducts of birds, reptiles and amphibians deposited in the whites of their eggs. In chicken egg white, avidin makes up approximately 0.05% of total protein (approximately 1.8 mg per egg). The tetrameric protein contains four identical subunits (homotetramer), each of which can bind to biotin (Vitamin B7, vitamin H) with a high degree of affinity and specificity. In its tetrameric form, avidin is estimated to be between 66–69 kDa in size.Ten percent of the molecular weight is attributed to carbohydrate content composed of four to five mannose and three N-acetylglucosamine residues. The carbohydrate moieties of avidin contain at least three unique oligosaccharide structural types that are similar in structure and composition.