t i t u p g s t ( uch as neurotransmitters, hormones, and light. The vast ajority of such receptors are coupled to G proteins, which ransduce the signal to effectors. G proteins are heterotrimrs made up of an a, a b, and a g subunit (29). The G protein ubunits are now known to be encoded by families of related enes in mammalian cells (74). The sizes of these mammaian gene families vary. a subunits are encoded by 16 genes, subunits are encoded by 5 genes, and g subunits are enoded by 12 genes. After the initial discovery of G proteins, most work focused n the a subunit. To identify the molecular basis for diversity n a subunit types, genes for this subunit were cloned and haracterized (41). Additional interest in the a subunit genes as stimulated by the discovery of mutations in these genes ssociated with human disease (76). In the past few years, everal genes for the b and g subunits have also been idenified and characterized (18, 19, 22, 38, 64, 73, 80). Informaion about these three gene families has also come from the arge-scale human and mouse genome sequencing efforts. he recently completed characterization of the genomes of accharomyces cerevisiae and Caenorhabditis elegans allows s to compare the size and extent of diversity of the mamalian G protein subunit gene families with that of a uniellular eukaryote and a lower invertebrate (http://www. cbi.nlm.nih.gov/PMGifs/Genomes/yc.html and http://www. cbi.nlm.nih.gov/PMGifs/Genomes/ce.html). We focus here n some aspects of the G protein subunit genes: (i) We sumarize the existent information on the structure, expression, nd organization of these genes. (ii) We focus on mutations in he genes for G protein subunits that result in disease and he molecular basis for their deleterious effect on cells. (iii) e discuss the implications of the increase in diversity and ize of the G protein subunit gene subfamilies with cellular omplexity. Recent reviews focusing on various related asects of G proteins are available (21, 27, 32, 62).