Marc’s early work focused on developing techniques for studying, purifying, and characterizing what are now known as G-protein-coupled receptors (GPCRs), using the β-adrenergic receptor as a model. He also contributed greatly to the discovery of the mechanisms by which receptor signaling is regulated by kinases and β-arrestins — in particular, how the β-arrestins are involved not only in desensitization but also in endocytosis and signaling. Early on, he developed a particular interest in dopaminergic signaling, which would lead to his laboratory’s developing tools for identifying, purifying and cloning the D1 and D2 receptors and clarifying their regulation and signaling mechanisms. In a classic paper, he defined an entirely new mechanism for regulation of behavior by D2 receptor signaling through β-arrestin.