Table 1.

Comparison of proposed intron gain mechanisms

Proposed mechanism of intron gain	Maintenance of protein size explained	Flanking repeats explained
DSB repair	Yes	Yes
Exon intronization	No (size loss)	No
Alu exonization	No (size increase)	Noa
Reverse splicing	Yes	No
Transposon insertion	?	Yes

^aComplete Alu repeats are usually flanked by direct repeats