Over-expression models have been generated to investigate the roles: of hormones in pituitary tumourigenesis, e.g. over-expression of the growth hormone releasing hormone (Ghrh) gene in mice resulted in pituitary adenomas that led to excessive GH secretion (Asa et al., 1992b); and of oncogenes and tumourigenic viruses. This evidence concerns the gene GH1 and pituitary gland adenoma.