Exonization of mobile genetic elements. A hypothetical gene locus is diagrammed (a) before and (b,c) after insertion of an L1 in antisense orientation. The L1 insertion allows for two alternative transcripts. (b) In one of these, the L1 sequence provides a novel transcription start site, an alternative first exon (ex1') and a splice donor site. (c) In the second, an entire new exon (ex3.1) with splice sites on either side is derived from the inserted L1. ex, exon.