Guanine nucleotide binding proteins (G proteins) that transduce signals from cell surface receptors to effector molecules are made up of three subunits, α, β, and γ. A complementary DNA clone that encodes a 71-amino acid protein was isolated from bovine brain; this protein contains peptide sequences that were derived from the purified γ subunit of Gi and G0. The primary sequence of this G protein γ subunit (Gγ) has 55 percent homology to the γ subunit of transducin (Tγ) and also has homology to functional domains of mammalian ras proteins. The probe for isolating the clone was generated with the use of the polymerase chain reaction (PCR). The extent of divergence between Tγ and Gγ, the isolation of homologous PCR-generated fragments, and the differences between the predicted amino acid sequence of Gγ and that derived from the γ subunit of Gi and G0 indicate that γ subunits are encoded by a family of genes.