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Group Ⅱ introns, which are bacterial elements, are ancestrally related to eukaryotic spliceosomal introns and retrotransposons.Spliceosomal introns and retrotransposons comprise over 50% of the human genome and are believed to be important in genome sculpting and stability.Group Ⅱ introns are mobile genetic elements that behave like retrotransposons in targeting specific DNA homing sites (retrohoming) or ectopic sites (retrotransposition).As retroelements, many of these introns encode reverse transcriptases.Like nuclear spliceosomal introns, group Ⅱ introns splice through successive transesterification reactions in which two nucleophilic attacks give rise to an excised intron lariat and ligated exons.