FGF-9 is a member of the fibroblast growth factor family. It is a potent mitogen and can stimulate proliferation and invasion capacity of epithelial cells. FGF-9 also plays an important role in lung development and maintenance in the adult organ. FGF-9 is expressed in the lung mesothelium and epithelium. FGF-9 regulates lung branching by stimulating mesenchymal proliferation and inducing FGF-10 expression in the mesenchyme. FGF-9 deficient mice exhibit severely impaired lung development and die in the perinatal period. FGF-9 deficient mice also exhibit other phenotypes including testicular hypoplasia and male-to-female sex reversion. Additional studies have shown that FGF-9 is required for normal sex determination by maintaining SOX9 expression. FGF-9 also regulates testosterone production in the mouse Leydig cell through AKT and MAPK signaling pathways. FGF-9 can be detected in the central nervous system and promotes the differentiation and survival of both neurons and glia cells. FGF-9 also regulates gap-junction formation in cultured astroglial cells. FGF-9 is also involved in bone formation, retinal differentiation and maturation, wound healing, and maintaining the stemness of kidney nephron progenitors. Expression of FGF-9 can be transcriptionally induced by prostaglandin E2, estrogen, and hypoxia. It has been shown that FGF-9 can form dimers and the dimeric FGF-9 binds to heparin with a higher affinity than the monomeric FGF-9. Overexpression of FGF-9 has been associated with many human cancers such as ovarian endometrioid adenocarcinomas, glioma, prostate cancer, and endometriosis.