Recent studies based on massively parallel sequencing techniques have contributed to identifying various types of noncoding RNAs (ncRNAs) in genomes including miRNAs, siRNAs, piwi-interacting RNAs (piRNAs), long noncoding RNAs (lncRNAs), and small nucleolar RNAs (snoRNAs), and they are known to be preferentially located in the intron regions within genes [46]. For instance, about half of the miRNAs in the human genome are located in introns, and they are usually co-expressed with their host genes regulated by the promoters of host genes [44]. Similar to miRNAs, some snoRNAs reside in introns, and they are also regulated by host transcriptional and splicing machineries [45]. Other ncRNAs, including lncRNAs and siRNAs, are also found in intron regions, though the proportion of lncRNAs and siRNAs in introns is lower than that of miRNAs and snoRNAs in introns [246]. Introns are classically degraded after the completion of splicing; however, these ncRNA genes embedded in intron regions are produced upon intron removal [246]. Furthermore, they can survive even longer than the intronic host genes [2]. Considering that the ncRNAs located in introns are co-expressed and co-regulated
