NR5A1 and Leydig cell tumor: Interestingly, while Nr2f2 is not expressed in non-proliferative mouse fetal Leydig cells, Nr2f2 is expressed in proliferative mouse Leydig cell tumor cell lines, such as MA-10 and MLTC-1, where it cooperates with Nr5a1 to drive the expression of steroidogenic genes52,59,60.