Since a compound heterozygous MHC risk haplotype effect has been described in SLE and RA (Shimane et al., 2013; Graham et al., 2007; Agrawal et al., 1995), further investigation in larger patient groups, including cohorts carrying compound heterozygous MHC risk alleles together with all homozygous cohorts was performed. The gene discussed is HLA-C; the disease is systemic lupus erythematosus.