The ectodomains of both the relaxin (LGR7) and the INSL3 (LGR8) receptors can be expressed on the cell surface using only a single transmembrane domain. These membrane-anchored proteins retain the ability to bind relaxin and can be cleaved from the cell surface. The subsequent LGR7 protein, 7BP, binds relaxin and can act as a functional relaxin antagonist. By contrast, the equivalent LGR8 protein 8BP does not bind relaxin or antagonize LGR8 activity. The 7BP protein has been successfully immobilized onto chemically derivatized surfaces for the capture of relaxin peptides and subsequent identification via SELDI-MS analysis.