PDCD1 and neoplasm: In conclusion, the bioactivity of chimeric anti-PD-1–GITR-L constructs has been demonstrated only in humanized single-target KI mouse tumor models, with no sign of bioactivity in WT mice, which not only validates PD biomarkers observed with the surrogate bispecific in WT mice but clearly suggests that target coengagement is crucial for the mechanism of action (MoA) of the bispecific.