Dopamine receptor sites in human growth hormone (GH)-secreting adenomas have been identified (38, 39), and heterodimerization of D2R and somatostatin receptor 5 (SSTR5) forms a Gi protein-linked effector complex (40) that may enhance functional activity, with interaction between the receptor/beta-arrestin complexes of the two receptor families affecting signaling and trafficking of activated receptors (41). This evidence concerns the gene DRD2 and adenoma.