Pre-mRNA permits one gene to encode more than one protein by alternative splicing, which leads to the production of multiple proteins called isoforms with various combinations of functional domains. FGFR2 a human gene that encodes the receptor that attaches the fibroblast growth factors and therefore, produces a signal within the cell. The FGFR2 (Fibroblast growth factor receptor 2) protein contains a number of domains, as well as an extracellular ligand-binding domain. The two isoforms that are produced by alternative splicing vary in their extracellular domains. Due to this variation, each isoform attaches to the dissimilar growth factors.
Based on the figure 9.19, the exon 8 exists in one isoform and exon 9 is present in another isoform. When a mutation occurs in 5' GU (Guanine and Uracil) in exon 8 would stop the splicing and obstruct the synthesis of its isoform. The mutation would not influence the splicing of exon 9 isoform. Similarly, a mutation in the 5' GU in exon 9 would inhibit the synthesis of its isoform only. Moreover, a nonsense mutation in any of the exons would stop the production of that particular isoform but not the other isoform.