HOXD13 and melanoma: In a separate study, circulating tumor-specific T cells were identified to be the prime source of sPD-1 as absent tumor-infiltrating lymphocytes (TILs) in melanoma patients was associated with high sPD-1 levels in plasma as opposed to brisk (TILs present across the entire base of the tumor) or non-brisk (TILs distributed only focally) TILs (153).