Recombinant Protein-A used also in Protein A beads (P9100-75) is a genetically engineered form of Protein-A derived from Staphylococcus aureus. Non-essential regions have been modified while leaving the IgG binding sites intact. Protein A is been used extensively used for the isolation of IgG from several species of mammals. The affinity of Protein A to IgG's is different between species and IgG subclasses within a given species. A major known limitation of protein A is its weak binding to Mouse IgG1