This invention relates to human fibroblast growth factor (hFGF-21), and to variants thereof and to polynucleotides encoding FGF-21. The invention also relates to diagnostic and therapeutic agents related to the polynucleotides and proteins, including probes and antibodies, and to methods of treating liver disease such as cirrhosis and cancer, methods of treating conditions related to thymic function, and methods of treating conditions of the testis. The invention also relates to mouse fibroblast growth factor (mFGF-21), and to variants thereof and polynucleotides encoding mFGF-21.