In general, both TEs and SSs are reported to be non-randomly distributed among eukaryotic genomes.1,21–23 On one hand, there is strong negative selection to protect essential sequences in genomes for the transmission of basic genetic information in a relative shorter evolutionary time scale, such as protein-coding sequences or exons. On the other hand, RSs are indispensable as the prime power and raw materials for genomes to evolve for better fitness, to generate complexity and diversity, and to promote speciation and population dynamics.2,24 Therefore, RSs have strong influences on gene expression and regulation indirectly through variations in intron length and content.10,13 One mechanism shared by all the studied vertebrates is that both TE and SS insertions increase intron size but the strength of the former is much greater than that of the latter. In fact, after eliminating RS insertions in all introns, we observed that the tendency of length increase in the four intron classes remains the same. In other words, the large introns remain large in size even without RS insertions in all four intron classes and so do small introns. However, the introns of anole and chicken genomes are exceptional, where the intron size definitions may shift or not be clearly distinguishable between large and small when RS insertions are removed from the intron sequences (data not shown). We observed a non-random and unbalanced expansion mechanism of intron size evolution: larger introns tend to grow faster than smaller ones when introns are enlarged to a certain size or over a specific threshold. Furthermore, we investigated relationship and mechanism of TE- or SS-driven intron expansions. Satellites can increase intron size at an early or primitive stage as they change intron size in a relatively limited scale, but transposons are capable of increasing intron size in a larger (such as LINEs) and more massive (such as LTRs in multiple insertions) scale and thus have stronger influence on intron size expansion. Most importantly, we observed a synergy between TE-driven and SS-driven insertions, providing a greater degree of intron expansion