Over time eukaryotic genomes have evolved to host genes carrying multiple exons 
separated by increasingly larger intronic, mostly non-protein-coding, sequences. 
Initially, little attention was paid to these intronic sequences, as they were 
considered not to contain regulatory information. However, advances in molecular 
biology, sequencing, and computational tools uncovered that numerous segments 
within these genomic elements do contribute to the regulation of gene 
expression. Introns are differentially removed in a cell type-specific manner to 
produce a range of alternatively-spliced transcripts, and many span tens to 
hundreds of kilobases. Recent work in human and fruitfly tissues revealed that 
long introns are extensively processed cotranscriptionally and in a stepwise 
manner, before their two flanking exons are spliced together. This process, 
called "recursive splicing," often involves non-canonical splicing elements 
positioned deep within introns, and different mechanisms for its deployment have 
been proposed. Still, the very existence and widespread nature of recursive 
splicing offers a new regulatory layer in the transcript maturation pathway, 
which may also have implications in human disease.