ALTERNATIVE EXONS. “In contrast, differentially regulated alternative exons have not been examined for common functional features. This is an important issue to address, since these exons are generally conserved and maintain frame. Moreover, because they comprise only 10%–30% of total alternative exons, they may possess important functional features that have been overlooked.”
http://www.sciencedirect.com/science/article/pii/S1097276512004820