Avidin is a basic charged glycoprotein present in the egg white and in some extent in tissues of various animals. Avidin is a homotetrameric protein that contains 4 identical subunits which bind to Biotin with high extent of affinity and specificity consisting of 128 amino acids and has a molecular mass of about 67K Daltons. Avidin-Biotin is a modified form of affinity purified avidin that combines the high affinity specific activity of the native egg white avidin to biotin molecule together with a very low background of the streptavidin produced by the bacteria Streptomyces avidinii.