However, recent findings suggest that low copy numbers of C4 are not an independent risk factor for SLE, but that they are related to other SLE risk loci such as HLA-DRB1*0301 (DR3 isotype) or HLA-DRB1*1501 (DR2 isotype) located inside the MHC.6 Indeed, the association of SLE and HLA-DRB1*0301 was observed in our patients and this finding is consistent with common genetic features of other Northern European lupus samples.26,27 In contrast, HLA-DRB1*1501 did not increase the risk for SLE in our cohort. Here, C4A is linked to systemic lupus erythematosus.