HLA-DRB1 and juvenile idiopathic arthritis: The most common and also the most clinically homogeneous categories of JIA, oligoarthritis and RF-negative polyarthritis, showed strong genetic correlation across the HLA region supporting our previous approaches of combining these categories for genetic studies.15 Combined analysis of these categories show they share association across the HLA region with strong association for HLA-DRB1 amino acid position 13.