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Historically, it has been assumed that the functional roles of G proteins in receptor recognition and effector regulation are specified by their diverse α subunits. However, the discovery of similarly diverse βγ subunits that participate in both of these functional processes has called this assumption into question; recent work suggests that G proteins function as heterotrimers whose roles in particular receptor signaling pathways are determined by their specific αβγ subunit combinations. Although much remains to be learned, the assembly of specific αβγ subunit combinations seems to involve both structural and spatial factors.