In solid phase binding assays, biotinylated Fe3+ Hb interacted with the lupus-associated autoantigens Sm, Ro52, Ro60, and RNP68k to varying degrees; interaction was significantly diminished upon competition with unlabeled Fe3+ Hb (Figure 4A, bottom panel), a strong indicator of the specificity of binding. Here, TRIM21 is linked to systemic lupus erythematosus.