Protein G, a bacterial cell wall protein isolated from group G Streptococci, binds to mammalian IgGs mainly through Fc regions. Native Protein G has three IgG binding domains and also the sites for albumin and cell-surface binding. Albumin and cell-surface binding domains have been eliminated from recombinant Protein G to reduce nonspecific binding. Protein G has greater affinity than Protein A for most mammalian IgGs, especially for certain subclasses including human IgG3, mouse IgG1 and rat IgG2a. Unlike Protein A, Protein G does not bind to human IgM, IgD and IgA.