Cinrebafusp alfa (PRS 343) is a high affinity CD137/HER2 bispecfic anticalin-based drug. Cinrebafusp alfa binds to recombinant human HER2 ( K d =0.3 nM) and human monomeric CD137 ( 4-1BB ; K d =5 nM). Cinrebafusp alfa facilitates T-cell costimulation by tumor-localized, HER2-dependent 4-1BB clustering and activation, further enhancing T-cell receptor-mediated activity and leading to tumor destruction. Cinrebafusp alfa has the potential for HER2+ solid tumors research.