The rat ob receptor gene has been isolated and cloned. Two different alleles have been identified: the wild-type, and the fa-allele which differs from the wild-type by only one base pair. The base pair change, however, introduces an MspI restriction site into the DNA sequence, and also results in an amino acid change. Also part of the invention are the novel receptors, vectors containing the nucleic acid encoding the receptors, host cells transformed with this gene, and assays which use the gene or protein and identify new ligands.