Two siblings, products of a consanguineous marriage, were markedly deficient in both albumin and IgG because of rapid degradation of these proteins, suggesting a lack of the neonatal Fc receptor, FcRn. FcRn is a heterodimeric receptor composed of a nonclassical MHC class I α-chain and β₂-microglobulin (β₂m) that binds two ligands, IgG and albumin, and extends the catabolic half-lives of both. Eight relatives of the siblings were moderately IgG-deficient. From sera archived for 35 years, we sequenced the two siblings’ genes for the heterodimeric FcRn. We found that, although the α-chain gene sequences of the siblings were normal, the β₂m genes contained a single nucleotide transversion that would mutate a conserved alanine to proline at the midpoint of the signal sequence. Concentrations of soluble β₂m and HLA in the siblings’ sera were <1% of normal. Transfection assays of β₂m-deficient cultured cells with β₂m cDNA indicated that the mutant β₂m supported <20% of normal expression of β₂m, MHC class I, and FcRn proteins. We concluded that a β₂m gene mutation underlies the hypercatabolism and reduced serum levels of albumin and IgG in the two siblings with familial hypercatabolic hypoproteinemia. This experiment of nature affirms our hypothesis that FcRn binds IgG and albumin, salvages both from a degradative fate, and maintains their physiologic concentrations.