Examples of NMD features. (a) Example of a gene upregulated in smg-1(r861) and showing transcript expression from the upstream ORF. (b) Exon4 of rsp-1 is alternatively spliced in smg-1(r861) animals, observed as a change in the NI of the exon between the two stages. (c) The retention of an intron expressed at very high levels in the mutant, which also has a higher gene intensity. (d) A retained intron, where the gene intensity remains similar between the mutant and wild type (that is, a gene for which only a small proportion of transcripts retain the intron).