1. A method for mapping the ligand binding domains of a protein which binds to a ligand, the method comprising the steps of (a) randomly cleaving the gene encoding for the protein into DNA fragments of sufficient length so as to likely include the ligand binding domain; (b) enzymatically adding only a single nucleotide residue overhang to one end of each strand of the DNA fragments from step (a); (c) inserting the DNA fragments from step (b) into a recombinant DNA construct having regulatory elements sufficient to transcribe and translate inserted DNA fragments in a host, the DNA construct having been prepared with single nucleotide overhangs complementary to the single nucleotide overhangs on the fragments from step (b); (d) transforming the DNA construct from step (c) into a host which transcribes and translates the recombinant DNA construct and culturing the host under conditions so that the DNA construct expresses peptides encoded by the DNA fragments; (e) screening the peptides for a peptide which binds to the ligand; (f) sequencing the DNA fragment which expresses the peptide which binds to the ligand; and (g) determining the corresponding amino acid sequence encoded by the DNA fragment of step (f), to thereby map the ligand binding domain of the protein.