Previous studies on sPD-1 have shown elevated serum and plasma levels and a correlation to disease activity in for example rheumatoid arthritis and systemic sclerosis, which is interesting in light of the observation that PD-1 deletion results in susceptibility to autoimmune disease in mice, but on the other hand sPD-1 was decreased in idiopathic thrombocytopenic purpura. This evidence concerns the gene HOXD13 and rheumatoid arthritis.