The FGF family has 22 members and is divided into 7 subfamilies based on their sequence similarity and phylogeny (So and Leung 2016). FGF21 belongs to an atypical subfamily, which lacks the conventional FGF heparin-binding domain, and thus FGF21 can diffuse away from the tissues of production and function as an endocrine factor (Ogawa et al. 2007). Nishimura’s team first cloned the human and mouse FGF21 genes in 2000 by means of PCR technique (Tetsuya Nishimura et al. 2000). Human FGF21 is located on chromosome 19 and mouse FGF21 is located on chromosome 7. Interestingly, the mammalian FGF21 gene is highly conserved; for example, mature FGF21 consists of 210 and 209 amino acids in mice and humans, respectively, and the two proteins share 75% of similarity of amino acid sequences (Tetsuya Nishimura et al. 2000).