TNFSF13B encodes the protein BAFF (B-cell activating factor), a cytokine primarily produced by monocytes and neutrophils, and involved in the development, survival and differentiation of B cells.110,111 The variant predisposing to autoimmunity results from the combination of a deletion with a polymorphism (GCTGT>A), referred to as BAFF-var. BAFF-var generates an alternative polyadenylation signal, leading to a mixed population of mRNAs characterized by long and short 3′ UTRs, in contrast to the wild-type allele, which produces only a long 3′UTR transcript. We observed that individuals carrying one copy of BAFF-var had a 35% reduction of the long transcript. Notably, the production of the shorter 3′UTR transcript was responsible for an increase of the RNA stability and translation due to the absence of a miRNA binding site for miR-15a, resulting in a rise of soluble BAFF. However, the increase in the mRNA expression explained only about 24 to 27% of the higher amount of soluble BAFF, indicating that an increase in translation level was also probably involved. The strong increase in the serum concentration of the soluble BAFF protein (about 19% per
