The structure and function of Human IGF-I are similar to insulin, but have higher growth-promoting activity than insulin. Human IGF-I expression is influenced by placenta lactogen, while Human IGF-I expression is regulated by growth hormone. Both Human IGF-I and IGF-II signal through the tyrosine kinase type I receptor (IGF-IR), but, IGF-II can also signal through the IGF-II/Mannose-6-phosphate receptor. Mature IGFs are generated by proteolytic processing of inactive precursor proteins, which contain N-terminal and C-terminal propeptide regions. Recombinant human IGF-I and IGF-II are globular proteins containing 70 and 67 amino acids, respectively, and 3 intra-molecular disulfide bonds.