It has been demonstrated that the majority of secreting and nonsecreting adenomas is monoclonal in origin suggesting that these neoplasia arise from the replication of a single mutated cell, in which growth advantage results from either activation of protooncogenes or inactivation of antioncogenes. Although a large number of genes has been screened for mutations, only few genetic abnormalities have been found in pituitary tumors such as allelic deletion of chromosome 11q13 where the MEN-1 gene has been localised, and mutations in the gene encoding the α subunit of the stimulatory Gs and Gi2 protein. These mutations constitutively activate the α subunit of the Gs and Gi2 protein by inhibiting their intrinsic GTPase activity. Both Gsα and Gi2α can be considered products of protooncogenes (gsp and gip2, respectively) since gain of function mutations that activate mitogenic signals have been recognized in human tumors. Gsp oncogene is found in 30–40% of GH-secreting adenomas, in a low percentage of nonfunctioning and ACTH-secreting pituitary adenomas, in toxic thyroid adenomas and differentiated thyroid carcinomas. The same mutations, occurred early in embriogenesis, have been also identified in tissues from patients affected with the McCune Albright syndrome. These mutations result in an increased cAMP production and in the subsequent overactivation of specific pathways involved in both cell growth and specific programmes of cell differentiation. By consequence, the endocrine tumors expressing gsp oncogene retain differentiated functions. The gip2 oncogene has been identified in about 10% of nonfunctioning pituitary adenomas, in tumors of the ovary and the adrenal cortex. However, it remains to be established whether Gi proteins activate mitogenic signals in pituitary cells. Since Gi proteins are involved in mediating the effect of inhibitory neurohormones on intracellular effectors, it has been proposed that in pituitary tumors the low expression of these proteins, particularly Gi1-3α, may contribute to uncontrolled pituitary cells growth by preventing the transduction of inhibitory signals. While by in vitro mutagenesis it has been demonstrated that activated mutant of Gqα, G12α, G13α and Gzα are fully oncogenic, it remains to be proved whether or not these abnormalities might naturally occur in human tumors and, in particular, in pituitary adenomas.