Alternative splicing provides one of the major mechanisms by which eukaryotic genomes can increase the diversity of transcripts and protein products encoded by a limited amount of genes. The spliceosome is involved in recognizing and removing intronic sequences and it is guided in this activity by splicing regulatory factors that determine the particular splice sites that are to be recognized and used for intron removal. Thus, by modulating the spliceosome's activity, these factors can cause different mRNA transcripts to be generated from the same precursor mRNA. In a recent RNAi screen for factors modulating RAS/MAPK signaling, we identified the Drosophila exon junction complex (EJC) as one of the components regulating the splicing of mapk transcripts. We showed that removal of EJC components caused multiple consecutive mapk exons to be skipped. Moreover, we determined that other fly genes that, like mapk, had particularly large introns, were also sensitive to disruption of the EJC. The importance of intron length in determining sensitivity to EJC disruption suggests that the EJC is functioning in exon definition, a splicing mechanism that occurs for long introns in particular.