The seven proposed mechanisms of intron gain and loss. Introns shown are shorter in length than necessary for splicing strictly for illustrative purposes. a) Intron Transposition with “partial” recombination. Other routes of intron transposition (discussed in text) by which concurrent intron gains/losses may occur can be also envisioned. b) Transposon Insertion. Imprecise intron gain may also be envisioned (discussed in text). c) Tandem Genomic Duplication using duplicated AGGT sequences for splice sites. The segment to be duplicated is flanked by brackets. The template for the duplicated nucleotides is highlighted in yellow; the duplicated nucleotides are highlighted in red and underlined. Imprecise intron gain may also frequently occur (discussed in text). d) Intron Gain during Double-Strand Break Repair. Protein names are from mammals. Short direct repeats created by gap filling are underlined. The possible pathways of non-homologous end joining (NHEJ) are from proposals in [45–48]. e) Insertion of a Group II Intron. f) Intron Transfer. g) Intronization. Strong consensus donor and acceptor sites (following a single point mutation) are highlighted in yellow. The place at which the hypothetical point mutation occurs is highlighted in red.