Figure 1.

Splicing and peptide products of the Igf1 gene. The Igf1 gene contains six exons; exons 1 and 2 (gray) serve as alternative leader exons at the 5′-end of the mRNA, whereas exons 3 and 4 (purple) are common to all splice variants. Several transcripts can be derived by alternative splicing at the C terminus, including exon 4 spliced directly to exon 6 (Ea) or exon 4 spliced to exon 5 spliced to exon 6. IGF-I mRNA containing exon 5 is referred to as Eb in rodents (Ec in humans) and has also been referred to as MGF (33). In humans, but not rodents, mRNAs containing exon 5 spliced to exon 4 have been identified (9) and are designated as Eb. Peptide products, derived from pro-IGF-I and referred to in the text, are shown.