Used in immunological research to study immune checkpoint pathways, particularly in cancer immunotherapy development. Serves as a binding partner for PD-1 receptor and helps in assays to evaluate T-cell activation and inhibition. Commonly employed in ELISA, surface plasmon resonance (SPR), and antibody screening to assess drug candidates targeting the PD-1/PD-L1 axis. The His tag facilitates easy purification and detection in recombinant protein applications.