HOXD13 and Autoimmunity: Several autoimmune disorders, such as rheumatoid arthritis [56], immune thrombocytopenia [57], systemic juvenile idiopathic arthritis [58] and pemphigus vulgaris [59] have been implicated with a defect in the PD1 pathway, whereby lower serum sPD1 levels were correlated negatively with the severity of the disease.