The recent cloning of the complementary DNAs and/or genes for several receptors linked to guanine nucleotide regulatory proteins including the adrenergic receptors (α₁, α_2A, α_2B, β₁, β₂)^1–7 several subtypes of the muscarinic cholinergic receptors^8,9, and the visual 'receptor' rhodopsin¹⁰ has revealed considerable similarity in the primary structure of these proteins. In addition, all of these proteins contain seven putative transmembrane α-helices. We have previously described a genomic clone, G-21, isolated by cross-hybridization at reduced stringency with a full length β₂- adrenergic receptor probe¹¹. This clone contains an intronless gene which, because of its striking sequence resemblance to the adrenergic receptors, is presumed to encode a G-protein-coupled receptor. Previous attempts to identify this putative receptor by expression studies have failed. We now report that the protein product of the genomic clone, G21, transiently expressed in monkey kidney cells has all the typical ligand-binding characteristics of the 5-hydroxytryptamine (5-HT_1A) receptor.