A novel proto-oncogene, PTTG (Pituitary Tumor Transforming Gene), was isolated in our laboratory by virtue of its increased expression in rat pituitary tumor cell lines. Cells which overexpress human or rat PTTG form tumors in athymic mice. hPTTG is highly expressed in cancer cell lines, pituitary adenomas and in normal testis, suggesting that hPTTG protein has different tissue-specific interactions in normal cells and in cancer. Alternatively, different hPTTG gene family members may be functional in normal development and in tumorigenesis. While mapping the chromosomal location of hPTTG to 5q33, we discovered a second gene, hPTTG2, which is intronless and maps to chromosome 4p12. Using gene-specific oligonucleotide hybridization in a PCR-ELISA assay, we determined that hPTTG2 is expressed in both normal and tumorous pituitary. However, high levels of hPTTG mRNA in cancer cell lines are due to increased expression of hPTTG1. Thus, this family of proto-oncogenes appears to differentially participate in tumor-specific pathogenesis.