Serum iron is ∼23% heritable (18), and increased concentrations have been associated with the A allele of the single nucleotide polymorphism (SNP) rs4820268 in transmembrane protease, serine 6 (TMPRSS6) (19, 20) and with the A allele of rs1800562 (C282Y) in hemochromatosis (HFE) (21), the former being common in many populations, whereas homozygosity of the latter is present in <0.5% of individuals of European ancestry (22) and is found in 80% of individuals with hemochromatosis (23), an iron overload syndrome. Here, TMPRSS6 is linked to hemochromatosis type 1.