In studying the age dependence and chronology of ovarian tumors in follicle stimulating hormone receptor knockout mice, we identified a novel ovarian tumor associated gene-12 (OTAG-12), which is progressively downregulated and maps to Chr. 8B3.3. OTAG-12 protein overexpression in mouse ovarian and mammary tumor cells suggested powerful anti-proliferative effects. In human epithelial ovarian cancers (OCs) and OC cell lines, OTAG-12 mRNA expression is downregulated in comparison with normal ovaries. Cloning and identification revealed that human OTAG-12 mapping to gene-rich Chr. 19p13.12 is expressed in three spliced forms: hOTAG-12a, hOTAG12b and hOTAG-12c, of which b is predominant in the normal ovary. Functionally active hOTAG-12b is a simple protein with no disulfide bonds and a nuclear localization signal is present in all variants. Transfection of OTAG-12 variants in OC and tumorigenic HEK293 cells confirmed nuclear localization. hOTAG-12b overexpression in OC and HEK293 cells effectively suppressed cell growth, anchorage-dependent and independent colony formation followed by apoptosis, whereas hOTAG-12a and hOTAG12c had no such effects. Deletion mutants identified the critical importance of carboxyl terminus for hOTAG-12b function. Doxycycline-inducible growth inhibition of HEK293 cells by hOTAG-12a was associated with effects on G2 cell cycle arrest and apoptosis induction. hOTAG12b expression rendered tumorigenic cells more sensitive to four apoptotic stimuli including etoposide-a topoisomerase-II inhibitor. Doxycycline-induced hOTAG-12b expression blocked xenograft tumor growth in nude mice, whereas hOTAG-12a was ineffective. Although p53-pathway-dependent apoptotic agents could upregulate endogenous hOTAG-12b and p53 in UCI-101/107 OC cells, hOTAG-12b could also induce apoptosis in p53-null and platinum-resistant SKOV3 OC cells and Doxycyclineinduced hOTAG-12b did not alter p53. Further study showed that hOTAG-12b increases mRNAs of proapoptotic genes such as BAD, GADD45a and CIEDB, while inhibiting anti-apoptotic NAIP and Akt1 expression, suggesting that hOTAG-12b-induced apoptosis might be p53-independent. These results indicate that hOTAG-12b is a putative ovarian tumor suppressor gene warranting further studies.