(A) Cellular SARS-CoV-2 3CLpro protein is exogenously expressed in HEK293T cells transfected with pcDNA3.1-Mpro plasmid. Proteins were separated by SDS–PAGE and visualised by Western blot. (B) Addition of untreated lysate (blue bars) from pcDNA3.1-Mpro (3CLpro expression vector) transfected HEK293T cells or 500 nM recombinant SARS-CoV-2 3CLpro (r3CLpro; orange bars) significantly increases net A625/A525 ratio compared with untreated lysate from pcDNA3.1 (control vector) transfected HEK293T cells or lysate treated with 1 µM GC376 (grey bars). Note: For Lysis Buffer + 1 µM GC376 samples, 500 nM r3CLpro was also added (error bars represent means ± standard deviation; n = 3 independent experiments; * P < 0.05, **** P < 0.0001 by two-way ANOVA Tukey's Multiple Comparisons Test). A representative visual image from independent replicate experiments is shown in a panel beside the graph.