Sequence analysis has revealed the presence of 31 copies of an extracellular domain, here called CUB, in 16 functionally diverse proteins such as the dorso-ventral patterning protein tolloid, bone morphogenetic protein 1, a family of spermadhesins, complement subcomponents Cls/Clr and the neuronal recognition molecule A5. Most of them are known to be involved in developmental processes. Our analysis of this new family includes the identification of seven previously undescribed members, the characterization of conserved features and a topology prediction of this approximately 110 residue spanning domain, which suggests an antiparallel beta-barrel similar to those in immunoglobulins.