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. This evidence concerns the gene TNFSF13B and systemic lupus erythematosus.