Surmounting an impasse of FcRn structure.

FcRn, resembling a major histocompatibility complex class I molecule with a closed peptide cleft, is an intracellular molecule that binds endocytosed albumin and IgG by a pH-dependent mechanism, diverting them from degradative fates and moving them out of the cell. The turnover of both of these important plasma proteins is thus regulated, as discussed by Schmidt and colleagues in this issue of Structure .