Other than whole genome duplication, the complexity of vertebrate genomes builds upon many unique sequence and functional features but one of them is genome expansion that compounds with the expansion of gene and intron sizes. There are three essential ways to increase genome sizes.18,19 The first is to increase the number of genes through genome and gene duplications. The second and also the foremost important mechanism is gene size expansion through intron size and number increases.20 The final way is the expansion of intergenic sequences and auxiliary chromosomal structures. With regard to the diversity of RSs and insertion/expansion mechanisms, we classified intron expansion into two categories: TE-driven and SS-driven,2,21 and speculated that they may play distinct roles in the intron size expansion of mammalian genomes. First, the profiles of TE insertions can be classified at levels of species and lineages, such as primates, large mammals, and rodents, and we did observe similar modes within lineages and distinctions among lineages. However, exceptions do exist as the rodents are not always cohesive—guinea pig behaves differently from mouse and rat concerning many RS counts. Second, we would like to emphasize the effect of RS expansion event rather than copy number counts, and we hope to see a clear and direct picture that correlates intron size variation with RS insertion.