Which is the target protein of the drug nivolumab?
['nivolumab, a fully human IgG4 programmed death 1 (PD-1) immune-checkpoint-inhibitor antibody', ' Nivolumab (a programmed death 1 [PD-1] checkpoint inhibitor)', 'Nivolumab, a fully human immunoglobulin G4 programmed death 1 immune checkpoint inhibitor antibody, ', ' programmed death receptor-1 (PD-1) inhibitors pembrolizumab and nivolumab were approved for the treatment of advanced melanoma', 'Nivolumab was developed as a monoclonal antibody against programmed death receptor-1, an immune checkpoint inhibitor which negatively regulates T-cell proliferation and activation. ', 'Nivolumab, a programmed death 1 (PD-1) checkpoint inhibitor,']
['Nivolumab was developed as a monoclonal antibody against programmed death receptor-1, an immune checkpoint inhibitor which negatively regulates T-cell proliferation and activation.']
['programmed death receptor-1']