Tumour necrosis factor (TNF) receptor family members regulate processes that range from cell proliferation to programmed cell death. The extracellular, ligand-binding domains of these proteins consist of small, cysteine-rich sub-domains, first observed in the three-dimensional structures of the type I TNF receptor. A structure-based alignment of TNFR family members indicates that the extracellular domains are constructed primarily of two small polypeptide modules. These modules play distinctive structural roles in the architecture of the domains. Analogues of at least one of these modules can be found in the domains of other receptors and extracellular proteins. Variations in their sequence and order of assembly are expected to account for differences in shape, flexibility and ligand specificity.