The invention provides a method for obtaining a binding compound for a protein family, wherein the members of the protein family bind a common ligand. The method includes the steps of (a) providing a sample containing a protein from the protein family, a first ligand and a second ligand under conditions wherein the first ligand, the second ligand and the protein form a bound complex; (b) detecting magnetization transfer between the first ligand and the second ligand in the bound complex, thereby determining that the two ligands are proximal in the bound complex; and (c) obtaining a candidate binding compound including the first ligand, or a fragment thereof linked to the second ligand, or a homolog thereof, whereby the population contains binding compounds that bind to members of the protein family.