 This difference is one basis for pharmacological biased agonism (also called functional selectivity), where a drug binding to a receptor may bias that receptor’s signaling toward a particular subset of the actions stimulated by that receptor.GRK2 is expressed broadly in tissues, but generally at higher levels than the related GRK3.GRK2 was originally identified as a protein kinase that phosphorylated the β2-adrenergic receptor, and has been most extensively studied as a regulator of adrenergic receptors (and other GPCRs) in the heart, where it has been proposed as a drug target to treat heart failure.