cARMs wereequipped with a monomeric rhamnose ABD to bind serumantirhamnose antibodies, an acyl imidazole electrophile, and an establishedglutamate urea (GU) TBD to target prostate-specific membrane antigen(PSMA, overexpressed 100–1000-fold in prostate cancer, GU ≈ Kd < 10–8 M).59,73−77 In this work, we observed that cARMs equipped with only a singlemonovalent rhamnose ligand can selectively covalently engage low-affinityrhamnose-specific serum antibodies. Here, FOLH1 is linked to prostate cancer.