document: Endotoxin assay. Briefly, 100 μL of 2-Mercaptoethanol (20 mM) was mixed with 100 μL of Qβ VLPs (10 μM) or 100 μL of endotoxin standard (100 EU/mL; Endotoxin, E. coli O55:B5, cat: 193783) at 37°C for 30 min followed by filtration using Amicon Ultra-0.5 Centrifugal Filter Unit (UFC5003, Millipore, USA). Then the endotoxin concentration in the elutriant (from Qβ VLPs or endotoxin standard) was then analysed by Kinetic-QCL™ Kinetic Chromogenic LAL Assay Kit.