A 3.5-μg aliquot of SARS-CoV-2 S protein as well as a 2-μg aliquot of human ACE2 were combined with Laemmli sample buffer, analyzed on a 4%–12% Invitrogen NuPage Bis-Tris gel using the MES pH 6.5 running buffer, and stained with Coomassie Brilliant Blue G-250.