FIGURE 1 Mobile group II introns encode the enzyme activities needed to propagate the intron to new areas of the genome and to new hosts. Two alleles of a gene (Y) differ by the presence (allele a) or absence (allele b) of an intron. In retrohoming, an enzyme called a retrotransposase is produced from a coding sequence within the excised intron. This transposase first cleaves gene Y, allele b DNA at the site of intron insertion, then copies the intron RNA into a cDNA copy that is ligated to create a new, intron-containing allele of gene Y.