HOXD13 and melanoma: In a study of patients with advanced melanoma, an increase in the sPD-1 concentration after the onset of treatment was a strong individual predictor of a better PFS for nivolumab plus ipilimumab, an antibody to cytotoxic T lymphocyte–associated protein–4 (CTLA-4), implicating sPD-1 in the activation of CD8+ T lymphocytes and the antitumor immune response (32).