FGF-8 belongs to the fibroblast growth factor (FGF) family and plays an important role in cell growth, embryogenesis, and tumorigenesis. Four isoforms of FGF-8 in human (a, b, e, f) and eight isoforms in mouse (a-h) generated by alternative splicing of the mRNA have been described. Comparing human and mouse, FGF-8a and FGF-8b show identical sequence homology. FGF-8b is predominant form and co-expressed with FGF-8a. The isoforms have different biological functions during embryogenesis. During prenatal stages, FGF-8 is required for the normal development of various organs, including limbs and central nervous system. Misexpression of FGF-8a and 8b proteins results in dysregulation of fate determination in developing brain. In addition, FGF-8 was first cloned from SC-3 mouse mammary carcinoma cells and was found to be induced in response to androgenic stimuli. The isoform FGF-8b has the highest affinity to FGF receptors, elucidating the stronger transforming capacity than FGF-8a. FGF-8b is detected in human prostate and breast carcinoma specimens and cell lines. FGF-8f is associated with the prognosis of esophageal carcinomas. FGF-8e mutation is associated with deficiency of gonadotropin-releasing hormone (GnRH).