Protein Sci. 2026 Oct;35(10):e70797. doi: 10.1002/pro.70797.ABSTRACTMultispecific, multivalent antibodies (Abs) are a burgeoning class of drugs that dramatically expand the pharmacological repertoire beyond traditional therapeutic Abs. Here, we present a simple, modular approach to developing multispecific, multivalent Abs based on a Fab-phage library with a single light chain. Using this library, we created three Abs targeting unique sites on programmed death-ligand 1 (PD-L1) and another antibody targeting CD30. Biophysical and cellular characterization of these Abs demonstrated their functional equivalence to clinically relevant Abs targeting PD-L1 or CD30. We then combined these paratopes into a series of bispecific, tetravalent, triparatopic Abs that retained the functionality of the parental Abs. Structural analysis of each of the Abs in complex with their cognate antigens demonstrated the adaptability of the common light chain to form diverse paratopes with an array of distinct heavy chains.PMID:42757708 | DOI:10.1002/pro.70797