Recombinant constructs encoding human and porcine beta-2 microglobulin Recombinant human β 2 m was expressed and purified as described elsewhere (Ostergaard et al. 2001) , (Ferre et al. 2003) . Using a previously reported E. coli codonoptimized gene encoding human β 2 m as template , a gene encoding porcine β 2 m was generated by multiple rounds of site-directed mutagenesis (QuikChange® Stratagene, according to the manufacturer's instructions) (Online Resource 2). Briefly, the genes encoding human or pig β 2 m were N-terminally fused to a histidine affinity encoding tag (HAT) followed by a restriction enzyme encoding tag (FXa), inserted into the pET28 vector and expressed in inclusion bodies in E. coli. The fusion proteins were extracted into 8 M urea, purified by immobilized metal affinity chromatography (IMAC), and refolded by dilution. The fusion tags were then removed by FXa restriction protease digestion. The liberated intact and native human or pig β 2 m were purified by IMAC and gel filtration chromatography, analyzed by SDS-PAGE analysis, concentrated, and stored at −20°C until use (Fig. 1 ).