ACE2 and cancer: This discrepancy can be caused by: (a) the difference between electrostatic interactions in 3D-folded and unfolded polypeptide chains; (b) the additional electric charges (ionized amino acid residues) of the polypeptide fragments attached to the C end of the used recombinant polypeptide chains (Fc tag for ACE2 and His tag for RBD); and (c) posttranslational modifications which can occur during the syntheses of recombinant proteins in cancer cell culture.