WNT, RAS or phosphoinositide 3-kinase signaling pathways control specific stages of ovarian follicular development. To analyze the functional interactions of these pathways in granulosa cells during follicular developmentin vivo,we generated specific mutant mouse models. Stable activation of the WNT signaling effector β-catenin (CTNNB1) in granulosa cells results in the formation of premalignant lesions that develop into granulosa cell tumors (GCTs) spontaneously later in life or following targeted deletion of the tumor suppressor genePten.Conversely, expression of oncogenic KRASG12D dramatically arrests proliferation, differentiation and apoptosis in granulosa cells, and consequently, small abnormal folliclelike structures devoid of oocytes accumulate in the ovary. Because of the potent anti-proliferative effects of KRASG12D in granulosa cells, we sought to determine whether KRASG12D would block precancerous lesion and tumor formation in follicles of the CTNNB1-mutant mice. Unexpectedly, transgenicCtnnb1;Kras-mutantmice exhibited increased GC proliferation, decreased apoptosis and impaired differentiation and developed early-onset GCTs leading to premature death in a manner similar to theCtnnb1;Pten-mutantmice. Microarray and reverse transcription-PCR analyses revealed that gene regulatory processes induced by CTNNB1 were mostly enhanced by either KRAS activation orPtenloss in remarkably similar patterns and degree. The concomitant activation of CTNNB1 and KRAS in Sertoli cells also caused testicular granulosa cell tumors that showed gene expression patterns that partially overlapped those observed in GCTs of the ovary. Although the mutations analyzed herein have not yet been linked to adult GCTs in humans, they may be related to juvenile GCTs or to tumors in other tissues where CTNNB1 is mutated. Importantly, the results provide strong evidence that CTNNB1 is the driver in these contexts and that KRASG12D andPtenloss promote the program set in motion by the CTNNB1.