The selectivity of PB-3 for wildtype PLY was evaluated by two approaches. First, a diverse collection of Michael acceptors which had been reported previously to target Cys-residues in other proteins or were derivatives of PB-3 was investigated as potential inhibitors of PLY in the hemolysis assay. None of these compounds inhibited PLY (Supplementary Figure and). Secondly, PB-3 was tested in assays of enzymes containing activated nucleophilic cysteine residues, namely the cysteine protease SARS-CoV2 main protease and cysteine-containing protein tyrosine phosphatase PTP1B without detecting significant activity of PB-3 (Supplementary Figure ). Deactivation of PB-3 by inactivated cysteine residues was not observed either, as indicated by the activity of PB-3 in the cellular assays, which were performed in Ham´s F12 medium containing 200 µM of Lcysteine, a 13,000-fold excess of free cysteine relative to the target protein PLY.