Amplification of SEI-1, a cell cycle regulatory gene at 19q13.1, is commonly detected in ovarian cancer, suggesting a role in the pathogenesis of ovarian cancer. In the present study, the oncogenic potential of SEI-1 was shown by anchorage-independent growth and tumor formation in nude mice with SEI-1-transfected NIH 3T3 mouse fibroblast cells. Silencing of SEI-1 gene expression by small interfering RNAs in ovarian cancer cell line SKOV3 could inhibit cell growth as well as colony formation on soft agar. Chromosomal alterations including the formation of double minutes were observed in tumor cells derived from SEI-1-transformed NIH 3T3 cells. Micronulei formation, which is an indicator of nuclear abnormality and genomic instability, was markedly increased in SEI-1-transfected cells. These data suggest that the oncogenic role of SEI-1 might be mediated at least in part via an effect on genomic instability. Furthermore, overexpression of SEI-1 was associated with higher tumor grades and late Fesddration Internationale des Gynaecologistes et Obstetristes (FIGO) stages in ovarian carcinomas. These data strongly suggest that SEI-1 plays an important role in the development and progression of ovarian cancer.