Alternative splicing can generate three different pro-IGF-I proteins, pro-IGF-IA, -IB and -IC. They share a common 16 amino acid N-terminal portion. Pro-IGF-IA has a further 19 amino acid extension not found in the other forms. Pro-IGF-IB has a 77 amino acid extension with some homology of a part of the 40 amino acid E-domain of Pro-IGF-IC (Duguay, S.J., 1999, Horm.MetabRes.31, 43-49). This antibody recognizes amino acids 104-110 of the E-domain of pro-IGF-IC.