OBJECTIVE: Interleukin-21 (IL-21) is a member of the type I cytokine superfamily that has a variety of effects on the immune system, including B cell activation, plasma cell differentiation, and immunoglobulin production. The expression of IL-21 receptor (IL-21R) is reduced in the B cells of patients with systemic lupus erythematosus (SLE), while serum IL-21 levels are increased both in lupus patients and in some murine lupus models. We recently reported that polymorphisms within the IL21 gene are associated with increased susceptibility to SLE. The aim of this study was to examine the genetic association between single-nucleotide polymorphisms (SNPs) within IL21R and SLE. METHODS: We genotyped 17 SNPs in the IL21R gene in 2 large cohorts of lupus patients (a European-derived cohort and a Hispanic cohort) and in ethnically matched healthy controls. RESULTS: We identified and confirmed the association between rs3093301 within the IL21R gene and SLE in the 2 cohorts (meta-analysis odds ratio 1.16 [95% confidence interval 1.08-1.25], P=1.0x10(-4)). CONCLUSION: Our findings indicate that IL21R is a novel susceptibility gene for SLE.