CR1 expression is downregulated by immune complexes and upregulated by interferon γ in SLE,37 and in the mouse, CR1/CR2 expression (encoded by the same gene) is upregulated by BAFF.38 Therefore, genetic differences in the compound regulation of CR1 expression are likely to be strongly influenced by the complex inflammatory disease milieu. Here, CR2 is linked to systemic lupus erythematosus.