FIGURE 5-16 Alternative splicing. The ~75-kb fibronectin gene (top) contains multiple exons; splicing of the fibronectin transcript varies by cell type. The EIIIB and EIIIA exons (green) encode binding domains for specific proteins on the surface of fibroblasts. The fibronectin mRNA produced in fibroblasts includes the EIIIA and EIIIB exons, whereas these exons are spliced out of fibronectin mRNA in hepatocytes. In this diagram, introns (black lines in the top diagram of the fibronectin gene) are not drawn to scale; most of them are much longer than any of the exons.