MRNA splicing (splicing) is a process to that removes introns from pre-mRNAs and ligates remaining regions termed exons to produce mRNAs. This reaction must be accurate, since insertion/deletion of nucleotides causes frame shift, which results in production of non-functional proteins. Signals for splicing mostly reside in introns. However, higher eukaryotes that harbor long introns in their genomes require additional splicing signals to modulate splicing appropriately. The additional signals in exons are termed ESE and ESS, exonic splicing enhancer and exonic splicing silencer, respectively. ESEs are bound to Serine-Arginine rich splicing factors (SRSFs) and promote exon recognition by recruiting essential splicing factors adjacent to splice sites. In contrast, ESSs can block exon recognition through binding to heterogeneous ribonucleoproteins (hnRNPs). This system, termed exon recognition, is a main system for splicing involving long introns in higher eukaryotes. The diverse sequences of splice sites and the large numbers and lengths of introns may have served to facilitate evolution of many splicing regulators such as SRSF proteins and hnRNPs in higher eukaryotes.