Mobility pathways mediated by homing endonucleases. Schematics of different endonuclease-mediated mobility pathways between donor and recipient alleles. (a) group I intron homing mediated by intron-encoded endonucleases; (b) the collaborative or trans homing pathway; (c) the intronless homing pathway mediated by free-standing endonucleases. In all cases, the homing endonuclease gene is represented by a green rectangle, and the homing site of the endonuclease is shown by a grey filled rectangle. The green rectangle outlined with dashed line indicates the outcome of a recombination event whereby the endonuclease ORF becomes embedded within an endonuclease-lacking intron, creating a potential mobile group I intron.