In skeletal muscle there is good evidence to suggest that locally produced insulin-like growth factor-1 (IGF-I), rather than circulating IGF-I, is important in regard to muscle mass maintenance, repair and hypertrophy. This "mature" IGF-I comprises exons 3 and 4 of the IGF-I gene, but during processing the full length gene (which contains six exons) is subject to a process of alternative splicing. As a result smaller peptides (E peptides) are believed to be cleaved from the mature IGF-I peptide during processing of the prohormone and the likelihood is that they have different biological roles. In human skeletal muscle three transcripts encoding for these splice variants (IGF-IEa, IGF-IEb and IGF-IEc, also known as MGF) can be identified. When studied at the mRNA level these three transcripts are known to be upregulated in the muscles of elderly people following high resistance exercise, albeit with different time courses. However, compared with mature IGF-I relatively little is known about the mechanism of action of the different E peptides.