To and 17B3 for MN1 (S/N = 2.9), representing a sandwich immunoassay for the smallest analyte ever described (Fig. and Supplementary Table ). The successful sandwich immunoassay for MN1, with an epitope distance of 2.4 Å and a molecular weight of 304 Da, demonstrated that the simultaneous binding of two mAbs to an extremely small analyte could be achieved through the careful selection and extensive pairing of mAbs.