We have designed a novel approach using genetically engineered mice to make a systematic study of the EDA exon regulation of the fibronectin gene during development and aging. The genome of the mice was modified either by optimization of the EDA natural splice sites or by deleting the EDA region. The previous in vitro observation that the optimization of the splicing sites leads to constitutive inclusion of the EDA exon was confirmed in our animal model. In fact, all the adult tissues of the genetically modified mice showed only EDA⁺ FN mRNA, demonstrating the fidelity of in vitro models, despite of the development- and aging-regulated splicing regulation of the EDA exon, and regardless of the presence of exonic elements described within the exon. This result indicates that the splicing regulatory elements of the EDA exon are dispensable in the presence of consensus splicing sites. Moreover, we demonstrate the autonomy of both the EDB and the IIICS alternatively spliced regions in adult mice lacking regulation of the alternative splicing at the EDA exon. We also show here the tight splicing regulation of all three alternative spliced regions of the FN gene at different time-points during development and aging of mice.