Consistent with the idea that cells in this primordium are poised between two developmental pathways, some of the genes that are involved in establishing sexual dimorphism, includingDax1 (dosage-sensitive sex reversal-congenital adrenal hypoplasia critical region on the X chromosome protein 1), Sox9 (Sry-like HMG box 9), Fgf9 (fibroblast growth factor 9), andWnt4 (wingless-related MMTV integration site 4), are initially expressed in similar patterns in XX and XY gonads [3–8]. The gene discussed is FGF9; the disease is chronic primary adrenal insufficiency.