Transposable elements (TEs) are major sources of new exons in higher eukaryotes. 
Almost half of the human genome is derived from TEs, and many types of TEs have 
the potential to exonize. In this work, we conducted a large-scale analysis of 
human exons derived from mammalian-wide interspersed repeats (MIRs), a class of 
old TEs which was active prior to the radiation of placental mammals. Using exon 
array data of 328 MIR-derived exons and RT-PCR analysis of 39 exons in 10 
tissues, we identified 15 constitutively spliced MIR exons, and 15 MIR exons 
with tissue-specific shift in splicing patterns. Analysis of RNAs from multiple 
species suggests that the splicing events of many strongly included MIR exons 
have been established before the divergence of primates and rodents, while a 
small percentage result from recent exonization during primate evolution. 
Interestingly, exon array data suggest substantially higher splicing activities 
of MIR exons when compared with exons derived from Alu elements, a class of 
primate-specific retrotransposons. This appears to be a universal difference 
between exons derived from young and old TEs, as it is also observed when 
comparing Alu exons to exons derived from LINE1 and LINE2, two other groups of 
old TEs. Together, this study significantly expands current knowledge about 
exonization of TEs. Our data imply that with sufficient evolutionary time, 
numerous new exons could evolve beyond the evolutionary intermediate state and 
contribute functional novelties to modern mammalian genomes.