Fig. 2  An example of three phases of intron from an Arabidopsis gene, AT1G17600.1. Upper/lowercase sequence indicates exon/intron sequence. Asterisks indicate frameshifts introduced by non-3n introns; intronic in-frame stop codons are underlined. Intron 1 is a 99-bp intron (3n) with one in-frame stop codon. Intron 2 is a 100-bp intron (3n + 2), which has two in-frame stop codons and thus does not interrupt the open reading frame. Intron 3 is a 74-bp intron (3n + 1) with three stop codons.