Effects of splicing errors under the intron definition (ID) and exon definition (ED) models. Arrowheads connected by horizontal bars illustrate the paired recognition of splice sites. (a) When splice sites are recognized in pairs across introns by ID, an error at a single splice site (marked 'x') prevents the removal of an intron, leading to a RI. Under ID, two adjacent splice sites must be mis-spliced, and the splicing machinery must operate over a greater distance, to generate a CE. (b) In the ED model, splice sites are recognized in pairs across exons. An error at a single splice site results in a CE. Obtaining an RI via ED requires coordinated mis-recognition of two adjacent splice sites over a longer distance. Observed RIs can be parsimoniously explained by ID-mediated splicing, while observed CEs likely indicate splicing via ED.