A new family of mosaic proteins is defined by sequence analysis. The family is characterized by a 260 residue domain common to proteins of apparently diverse function and tissue specificity: sperm receptors Zp2 and Zp3, betaglycan (also called TGF-beta type III receptor), uromodulin, as well as the major zymogen granule membrane protein (GP-2). The location of the common domain is similar with respect to putative transmembrane regions. The results lead to the hypothesis that this type of domain has a common tertiary structure and that there is a functional similarity in the recognition mechanism of the sperm receptor system and the TGF-beta receptor complex.