Possible mechanisms for the gain of protein domains. Colored blocks represent exons, with red, blue and green indicating exons coding for different domains. Solid black lines represent introns and red lines indicate intergenic regions. (a) Gene fusion. The noncoding region between two genes is modified so that the exons of the first gene become spliced with the second. (b) Exon extension. The noncoding region following an exon becomes part of the exon and codes for a new domain. (c) Exon recombination. The exons of two genes become directly joined. (d) Intron recombination. An exon from one gene is inserted into the intron of another. (e) Retroposition. A retrotransposon sequence (RT, purple) mediates the copying of itself and a neighboring gene region via an mRNA intermediate, followed by insertion into another gene.