To be considered in this discussion is a family of guanine nucleotide-binding proteins (G proteins) that serve as membrane-bound transducers of chemically and physically coded information. Knowledge of this family, particularly that acquired over the past 10 years, permits a rather restrictive definition of the characteristics of its closest members. I present such a definition here in the interest of generaliz ation and describe these characteristics in more detail throughout this review. However, we must remain alert for devi ations from the" rules" that may be practiced by as yet undiscovered members of the immediate family or by other related proteins.