Fibroblast Growth Factor 8 was originally identified as an androgen-dependent growth of mouse mammary carcinoma cells. The primary transcript of mouse FGF-8 is alternatively spliced to generate at least 8 secreted isoforms that differ at their amino terminus. The differences between the isoforms exist in the number of potential N-linked glycosylation sites. In mouse, the eight isoforms are labeled as 8a through 8h. Human FGF-8 is limited to only four isoforms. Only isoforms 8a, 8b, 8e, and 8f are synthesized in humans. Mouse and human 8a and 8b isoforms are 100% identical, while the 8e and 8f isoforms are 98% identical. The FGF-8 isoforms differentially activate the various alternatively spliced forms of the FGF receptors 1-3, and FGF receptor 4. The isoform FGF-8c activates FGF R3c and FGF R4.