The neurotransmitter-aminobutyric acid (GABA) exerts its effects through an ion channel, GABAA, and a GPCR, GABAB. Functional GABAB is a heterodimer composed of the GABAB1 and GABAB2 subunits, which share 35% sequence identity and belong to the class 3 family of GPCRs. The GABAB1 subunit, which exists as splice variants GABAB1a and GABAB1b, binds directly to GABA and is required for agonist activation. The GABAB2 and GABAB1 subunits associate by formation of a coiled coil by their C-terminal tails; this association masks an ER retention sequence in GABAB1 to permit export from the ER and trafficking to the cell surface. In addition to its chaperone function, GABAB2 is the component that couples to Gi to reduce intracellular cAMP. Agonists of GABAB, such as baclofen, are used clinically for treatment of muscle spasticity, migraine headache and musculoskeletal pain.