HLA-DRB1 and mixed connective tissue disease: The risk diplotype HLA-DRB1*09:01-DQB1*03:03/DRB1*15:01-DQB1*06:02 would encode DQA1*01:01-DQB1*06:02 and DQA1*01:02-DQB1*03:03 molecules in trans. Other culprit genes in linkage disequilibrium with HLA-DRB1-DQB1 loci could also cause MCTD.