HLA-DRB1 and systemic sclerosis: The frequencies of both HLA-DPB1*13:01 and HLA-DRB1*11:04 alleles were significantly increased in ATA + relative to ATA −disease (OR = 7.85 and OR = 4.13 respectively), however neither differed in frequency from controls in ATA − or ACA + SSc cohorts, indicating the strong specificity of these genetic association for patients with ATA + serology.