FOXL2 is a critical factor essential for ovary differentiation and maintenance and repression of the genetic program for somatic testis determination. It prevents trans-differentiation of the ovary to testis through transcriptional repression of the Sertoli cell-promoting gene SOX9. FOXL2 is mainly expressed in the ovary, parathyroid gland, and sex cord-stromal tumors. FOXL2 is generally used to diagnose sex cord-stromal tumors with inhibin, calretinin, and SF-1.