The underlying molecular pathogenic mechanisms remain unknown in the majority of human pituitary tumors. GADD45γ is a member of a growth arrest and DNA damage-inducible gene family that functions in the negative regulation of cell growth. We have found that the mRNA expression of the GADD45γ gene is significantly different between normal human pituitary tissue and clinically nonfunctioning pituitary adenomas using cDNA-representational difference analysis. Although GADD45γ mRNA was found in normal human pituitary tissue, it was detectable in only 1 of 18 clinically nonfunctioning pituitary tumors by RT-PCR. Furthermore, this gene was not expressed in the majority of GH- or PRL-secreting pituitary tumors (6 of 8 and 7 of 10, respectively). In colony formation assays, transfection of human GADD45γ cDNA into the human pituitary tumor-derived cell line, PDFS, results in a dramatic decrease in cell growth by 88%. GADD45γ also reduces colony formation in other pituitary tumor-derived cell lines, AtT20 and GH4, by approximately 60% and 50%, respectively, confirming its function in controlling cell proliferation in the pituitary. These data indicate that GADD45γ is a powerful growth suppressor controlling pituitary cell proliferation, and GADD45γ represents the first identified gene whose expression is lost in the majority of human pituitary tumors.