Epistasis in a Fitness Landscape Defined by Antibody-Antigen Binding Free Energy

？ Epistatic effects are extracted from massive measurements of binding energies ？ Variability in binding is 62% due to additive effects and 25%–35% due to epistasis ？ Up to 4% of double mutants have sign epistasis ？ Simulations suggest that epistasis may expand paths of affinity maturatio