G proteins link heptahelical membrane receptors to their effector systems. The G proteins consist of three subunits, α, β, and γ, of which until now 23 (including splice variants), 6, and 11 different forms are known, respectively (for reviews, see refs. 1 and 2 ). By sequence homology of G protein α-subunits, they are divided into four subfamilies, Gs , G1 , Gq , and G12 . Agonist bindmg is assumed to induce a conformational change of the receptor, which causes exchange of GDP for GTP at the G protein a-subumt, dissociation of Gαβγ from the activated receptor and dissociation of the Gα-GTP complex from the Gβγ dimer. Both activated Gα-GTP and free Gβγ have the capability of interacting with different effecters, e.g., adenylyl cyclases, phospholipases Cβ (PLCβ), and ion channels. It is of growing interest to establish whether receptors may be able to select among individual α-, β-, and γ-subunits for coupling to specific effector systems.