Assay recognition of different Protein A constructs, post boiling. Resulting concentrations were interpolated from kit standard curve. Percent recovery calculated by dividing observed concentration by expected concentration. A: Natural Protein A from S. aureus (Millipore), n=9; B: Recombinant Protein A from E. coli (Repligen), n=9; C: Recombinant Cys-Protein A from E. coli (GE), n=12; and D: Recombinant alkaline-resistant Protein A variant from E. coli (MabSelet SuRe from GE), n=12; graphical data represents statistical mean +/- 1 standard deviation.