Little is known about the pathogenesis of ovarian granulosa cell tumors. Recently, we reported the identification of a somatic FOXL2 402C-->G mutation that is present in virtually all adult-type granulosa cell tumors, but not in a wide range of other tumor types. This finding has important implications for the diagnosis and classification of ovarian sex cord-stromal tumors, provides insight into the pathogenesis of adult-type granulosa cell tumors, and opens possibilities for the development of targeted therapies.