HLA-DRB1*04:01 allele carrier frequencies in SLE or SSc patients with anti-U1RNP antibodies were analyzed, to determine whether DRB1*04:01 was primarily associated with the production of anti-U1RNP antibodies or with susceptibility to MCTD (Supplementary Table S5). This evidence concerns the gene HLA-DRB1 and systemic lupus erythematosus.