Soluble hemojuvelin is present in human serum and plasma. Serum and plasma samples were separated on reducing SDS-PAGE. In Western blot analysis of all serum samples, Ab112 detected a protein band (^*) of 30 kDa (3 different donors nos. 1-3, left panel), and anti-G3pep2-2 antibody detected a protein band (^**) of 16 kDa (middle panel). Pretreatment of Ab112 with excess rs-hemojuvelin abolished the 30-kDa Western blot signal (Figure S3). Blood plasma (1 μL, P, right panel) probed with Ab112 contained bands identical to those of serum from the same donor (1 μL, S, right panel) indicating that the hemojuvelin cleavage was not caused by the clotting reaction.