FGF-4 was first identified as HST-1 gene by a NIH3T3 transforming assay. It belongs to the FGF family that includes 22 members FGF1-FGF23. FGF15 has not been identified in humans. FGF-4 exhibits strong effects on many different cell types and tissues, and plays important roles in both embryogenesis and adult tissue stem cell development in a variety of organisms. FGF-4 can stimulate limb mesenchyme proliferation and can provide all the signals required for normal outgrowth and patterning of the limb. FGF-4 mRNA has been detected in the apical ectodermal ridge of the limb bud in Days 11 and 12 embryos. FGF-4 is also involved in early heart development by supporting the proliferation and differentiation of precardiac myoblasts. In adults, FGF-4 is expressed in the testis and its overexpression results in enhanced spermatogenesis. FGF-4 is also a potent inducer of platelet production from megakaryocytes. FGF-4 expression has been detected in the brain at both neonatal and adult stages. FGF-4 can induce neural stem cell proliferation and neuronal differentiation. The diverse functions of FGF-4 are mediated by FGF receptors (FGFR) that contain an extracellular ligand-binding domain with three immunoglobulin-like domains. Like other FGFs, FGF-4 also binds to anionic glycosaminoglycans heparin and heparin sulfate with high affinity. Heparin sulfate availability has been shown to regulate the binding between FGF-4 and its receptors. The role of FGF-4 in cancer has also been extensively investigated. FGF-4 is an angiogenic protein, and amplification of FGF-4 gene has been found in many human tumors. It has been shown that FGF-4 is a potential target for the treatment of human testicular tumors.